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Codeine
3-methylmorphine
Codeine (3-methylmorphine) is a naturally-occurring opioid of the morphinan class. It is the chemical precursor to morphine — a prodrug the body converts to morphine to produce its effects.[1][2] Codeine itself binds opioid receptors only weakly;[3] it is morphine that suppresses pain and produces the sedation and warmth characteristic of opioids.
Codeine (3-methylmorphine) is a naturally-occurring opioid of the morphinan class. It is the methyl ether of morphine, functioning primarily as a prodrug.[1][2] Codeine is a weak direct µ-opioid receptor agonist,[3] producing its principal effects through morphine-mediated suppression of pain signaling and respiratory drive.
+1 more
Depressantmorphine, heroin, oxycodone +4 moremorphine, heroin, oxycodone, hydrocodone, fentanyl, tramadol, dihydrocodeine show less
Codeine was isolated from opium in 1832 by French chemist Pierre-Jean Robiquet.[4] It became one of the most prescribed opioids globally before fatal pediatric cases linked to genetic variation in metabolism prompted an FDA black box warning in 2013.[5] Regulatory access has tightened since — Australia moved codeine to prescription-only in 2018,[6] and the trend continues worldwide.
Subjective effects include pain suppression, sedation, mild euphoria, anxiolysis, a sense of diffuse warmth, and cognitive slowing. The overall experience is a quieter version of morphine — gentle and tranquil — but its intensity varies dramatically between individuals based on how efficiently genetics convert codeine to morphine.[1] Nausea, constipation, and drowsiness are characteristic unsought effects.
Codeine produces moderate physical and psychological dependence, and its main toxicity risk is respiratory depression driven by morphine.[7] The defining danger is genetic: people who convert codeine to morphine unusually fast can experience life-threatening effects from standard doses — a trait present in up to 18% of some populations.[8] Codeine is among the most thoroughly studied opioids, with clinical data spanning nearly two centuries and a major pharmacogenomic trial published in 2025.[9]
History[edit]
Discovery
Codeine was isolated from opium in 1832 by French chemist Pierre-Jean Robiquet while refining methods for extracting morphine from the opium poppy.[4] The name comes from the ancient Greek kodeia — "poppy head." It was the first naturally occurring opium alkaloid characterized independently of morphine, opening the door to more predictable analgesic and cough-suppressing formulations.
Clinical Adoption and Regulation
Codeine became one of the most prescribed opioids globally, eventually recognized as an essential medicine by the World Health Organization. Fatal pediatric cases were linked to CYP2D6 ultrarapid metabolizer status,[5] leading the FDA to issue a black box warning in 2013 barring use in children under 12 and post-tonsillectomy patients.
Modern Restrictions
Australia's 2018 rescheduling of codeine from over-the-counter to prescription-only was the most significant recent regulatory shift, driven by evidence of widespread misuse and dependence.[6][10] Codeine remains controlled internationally under the 1961 Single Convention on Narcotic Drugs. The global trend is toward tighter restrictions on low-dose over-the-counter preparations.
The recreational preparation known as "purple drank" — codeine-promethazine cough syrup mixed with soft drinks — originated in Houston, Texas in the 1960s and gained wide visibility through hip-hop culture in the 1990s and 2000s.[11][12] The promethazine component adds sedative effects that compound codeine's opioid action, and promethazine misuse in this context is independently associated with drug-related deaths.[13]
Chemistry[edit]
Codeine belongs to the morphinan class — tetracyclic alkaloids derived from the opium poppy Papaver somniferum, where it constitutes roughly 2% of crude opium by weight.[4] It is the 3-O-methyl ether of morphine: one methyl group is the sole structural difference between the two compounds.
That single modification strips most of codeine's direct opioid receptor affinity, making it a prodrug requiring metabolic conversion back to morphine to act. Its molecular formula is C₁₈H₂₁NO₃ (299.4 g/mol),[14] and in the plant it serves as the penultimate step in the biosynthetic route to morphine.[15]
Pharmacology[edit]
Mechanism of Action
Codeine itself binds opioid receptors only weakly — roughly 100 to 250 times less potently than morphine.[16] Its pharmacological activity comes almost entirely from morphine, produced when a liver enzyme (CYP2D6) removes its methyl group.[1][2] Only about 10% of a dose follows this conversion pathway; the majority becomes codeine-6-glucuronide, a metabolite with weak opioid activity.
Antitussive and Gastrointestinal Effects
Codeine suppresses cough by activating opioid receptors in the brainstem's cough center — an effect that naloxone completely abolishes, confirming it is opioid-mediated.[17][18] Separately, opioid receptor activation in the gut slows intestinal movement, producing constipation even at therapeutic doses.[19] This gut effect is peripheral — independent of the brain.
Pharmacokinetics
Codeine is well absorbed orally — over 90% of a dose reaches the bloodstream,[14] with effective bioavailability around 50–60% after first-pass liver metabolism.[20] Peak blood levels arrive within about 1–1.4 hours of a 60 mg oral dose, with an half-life of approximately 2.8 hours.[21] The liver processes codeine through CYP2D6, CYP3A4, and UGT2B7.[22]
Neuroscience[edit]
Where It Acts
All of codeine's neural effects run through morphine, its active metabolite, acting at opioid receptors across several brain regions. In the brain's reward circuit, morphine triggers dopamine release, producing euphoria. In the brainstem, it suppresses the cough reflex and activates pain-inhibiting pathways.[17][18] The strength of every one of these effects scales directly with how much morphine the body makes — which depends on CYP2D6 genetics.
Why It Matters
Repeated opioid receptor activation leads to tolerance: the brain dials down its response, and higher doses become needed for the same effect. Chronic exposure also suppresses the body's own opioid system, contributing to withdrawal when use stops. Codeine makes this dynamic unusually visible because the same pill produces fundamentally different brain effects depending on a person's genetic enzyme profile.
Codeine-6-glucuronide — the compound accounting for 70–80% of a dose — has weak opioid activity,[23] but whether it enters the brain in meaningful amounts is unknown. Codeine is also more convulsant than morphine in animal studies,[1] though the relevance to humans at normal doses remains unclear. Population pharmacokinetic models have mapped metabolite ratios in blood,[22] but brain-level predictions are untested.
Subjective Effects[edit]
Codeine produces a diffuse warmth and quiet sedation — a soft settling of physical and emotional discomfort. The defining quality is what it removes rather than what it adds: pain recedes, anxiety fades, and the body grows heavy and still. The intensity is genuinely unpredictable, shaped more by individual genetics than by dose alone.
Compared to morphine, codeine produces a qualitatively similar but noticeably lighter experience — less euphoria, less sedation, and reduced respiratory depression. Relative to tramadol, codeine lacks serotonergic properties, producing a cleaner opioid profile without the stimulant-like qualities tramadol can bring at higher doses.
Isolated from opium in 1832 by Pierre-Jean Robiquet,[4] codeine became one of the most widely prescribed opioid analgesics and antitussives globally. Fatal pediatric cases linked to genetic variation in codeine metabolism prompted an FDA black box warning in 2013,[5] and Australia rescheduled codeine to prescription-only in 2018.[6] Regulatory scrutiny continues to tighten access worldwide, particularly for over-the-counter formulations.
Subjective effects include analgesia, sedation, anxiolysis, euphoria, and cognitive slowing. The experience is one of diffuse warmth and mild tranquility — a muted version of morphine's signature calm, whose intensity varies dramatically between individuals based on genetic metabolizer status.[1] Nausea, constipation, and drowsiness are characteristic unsought effects.
Codeine produces moderate physical and psychological dependence, with toxicity driven primarily by morphine-mediated respiratory depression.[7] The defining risk is pharmacogenomic: individuals who metabolize codeine ultrarapidly can experience life-threatening effects from standard doses — a trait present in up to 18% of some populations.[8] Codeine is well-characterized through decades of clinical use, with the largest prospective pharmacogenomic trial published in 2025.[9]
History[edit]
Discovery
Codeine was isolated from opium in 1832 by French chemist Pierre-Jean Robiquet while refining methods for extracting morphine from Papaver somniferum latex.[4] The name derives from the ancient Greek kodeia (κωδεία, "poppy head"). As the first naturally occurring opium alkaloid characterized independently of morphine, codeine opened a path toward antitussive and analgesic formulations with more predictable dosing than crude opium.[4]
Clinical Adoption and Regulation
By the late nineteenth century, codeine was in widespread clinical use for pain, cough suppression, and diarrhea — eventually becoming one of the most prescribed opioids globally and earning recognition as an essential medicine by the World Health Organization. Regulatory action intensified in the 2010s after pediatric fatalities were linked to CYP2D6 ultrarapid metabolizer status.[5] The FDA issued a black box warning in 2013 contraindicating codeine in children under 12, post-tonsillectomy patients, and breastfeeding mothers.
Modern Restrictions
Australia's 2018 rescheduling of codeine from over-the-counter to prescription-only marked the most significant recent regulatory shift, driven by evidence of widespread OTC codeine misuse and dependence.[6][10] The trend across jurisdictions is toward tighter control of low-dose codeine preparations. Internationally, codeine is controlled under the 1961 Single Convention on Narcotic Drugs (Schedule II).
The name codéine was coined by Robiquet from the Greek kodeia (poppy head), reflecting the compound's botanical origin rather than its pharmacology. Codeine is sometimes called a "weak opioid" in clinical parlance, though this characterization obscures the pharmacogenomic variability that makes it unpredictably potent in some individuals.
Chemistry[edit]
Structural Class
Codeine belongs to the morphinan structural class — tetracyclic alkaloids defined by a 6,6,6,5-fused ring system. It is the 3-O-methyl ether of morphine: the sole structural difference is methylation of morphine's phenolic 3-hydroxyl group to a methoxy group. This single modification dramatically reduces direct opioid receptor affinity and renders codeine a prodrug requiring metabolic removal of that methyl group (O-demethylation) to regenerate active morphine.
Codeine is structurally related to Morphine, thebaine, and the semi-synthetic opioid hydrocodone. It is one of approximately 80 alkaloids found in Papaver somniferum.
Molecular Properties
| Molecular formula | C₁₈H₂₁NO₃ |
| Molecular weight | 299.4 g/mol |
The morphinan skeleton comprises a phenanthrene core (rings A, B, C) fused to a piperidine nitrogen-containing ring (ring D) and an oxygen-bridged furan ring (ring E). The 3-methoxy group on ring A distinguishes codeine from morphine; the 6-hydroxyl group and 7,8-unsaturation are metabolically significant.
Stereochemistry
Codeine possesses five chirality centers. The naturally occurring (−)-codeine enantiomer is the pharmacologically active form; the unnatural (+)-codeine enantiomer lacks opioid receptor affinity. All pharmaceutical preparations contain exclusively the natural (−)-isomer.
Natural Occurrence
Codeine occurs naturally in the latex of Papaver somniferum (opium poppy), constituting approximately 2% of crude opium by weight.[4] In the plant, codeine is biosynthesized via the benzylisoquinoline alkaloid pathway from tyrosine precursors and serves as the penultimate intermediate in the biosynthetic route to morphine.[14] Commercial codeine is obtained either by extraction from opium or by semi-synthesis from morphine or thebaine.
Physical Properties
Codeine free base appears as white, odorless, fine needles or crystalline powder.[15] It is classified as BCS Class I (high solubility, high permeability).[16] The phosphate salt (codeine phosphate hemihydrate) is the most common pharmaceutical form due to its superior aqueous solubility.
Pharmacology[edit]
Mechanism of Action
Codeine functions primarily as an opioid prodrug. Its own binding to µ-opioid receptors is weak — roughly 100–250 times less potent than morphine.[17] The pharmacological activity attributed to codeine is overwhelmingly mediated by its active metabolite morphine, produced by a liver enzyme (CYP2D6) that removes the methyl group at the 3-position.[1][2]
An oral dose of codeine follows three metabolic pathways. Approximately 10% is converted to morphine (the active analgesic metabolite) by CYP2D6. Another ~10% is converted to norcodeine (inactive) by CYP3A4. The majority (~70–80%) is converted to codeine-6-glucuronide by UGT2B7, a metabolite with weak opioid activity.[2]
Morphine produced from codeine undergoes further metabolism to morphine-6-glucuronide (a potent opioid contributing to analgesia) and morphine-3-glucuronide (inactive). The ratio of these metabolites determines the clinical profile of any given dose.
Antitussive Mechanism
Codeine suppresses cough through µ-opioid receptor activation in the brainstem cough center. This has been localized by studies showing that brainstem-directed administration suppresses cough while spinal administration does not.[18][19] Naloxone (an opioid antagonist) completely abolishes the antitussive effect, confirming the opioid mechanism.[19]
Gastrointestinal Effects
Opioid receptor activation in the gut's nervous system slows intestinal movement, producing constipation even at therapeutic doses.[20] This is a peripheral effect independent of the central nervous system.
Pharmacokinetics
Codeine is well-absorbed orally, with over 90% of a dose taken up from the gastrointestinal tract.[16] After first-pass liver metabolism, effective oral bioavailability is approximately 50–60%.[21] Peak blood levels are reached within about 1–1.4 hours of a 60 mg oral dose, and the half-life is approximately 2.8 hours.[22]
Codeine is approximately 30% bound to plasma proteins.[22] It is eliminated primarily through the kidneys as glucuronide conjugates.[16] Primary metabolism occurs in the liver via CYP2D6, CYP3A4, and UGT2B7.[23]
Neuroscience[edit]
Where It Acts
Codeine's neural effects are mediated through morphine (its active metabolite) acting at µ-opioid receptors across several brain systems. In the ventral tegmental area, morphine disinhibits dopamine neurons projecting to the nucleus accumbens, producing the rewarding and euphoric properties of the drug. In the periaqueductal gray and brainstem, morphine activates descending pain-inhibiting pathways and suppresses cough reflexes.[18][19] The magnitude of all these effects is directly proportional to the amount of morphine produced — which varies dramatically with individual CYP2D6 genetics.
Why It Matters
Codeine exemplifies prodrug-dependent pharmacology: the same pill produces fundamentally different neurological effects depending on the user's genetic enzyme profile. Poor metabolizers experience minimal opioid receptor occupancy in the brain, while ultrarapid metabolizers may achieve morphine levels comparable to direct morphine administration. This makes codeine a natural model for studying how pharmacogenomics shapes drug response, reward, and risk at the neural level.
Repeated µ-opioid receptor activation produces tolerance through downregulation, internalization, and cellular counter-adaptation — processes that collectively reduce response to a given dose and necessitate escalation to maintain effect. Chronic opioid exposure also downregulates the endogenous opioid system (endorphins, enkephalins), contributing to dependence and withdrawal.
Whether CYP2D6 genotype affects central opioid receptor occupancy independently of blood morphine levels remains unconfirmed — population pharmacokinetic modeling has characterized peripheral metabolite ratios,[23] but brain-level predictions are untested. The contribution of codeine-6-glucuronide (70–80% of the dose) to clinical effect is also poorly understood: it has weak opioid activity,[24] but its brain penetration is unknown.
Subjective Effects[edit]
Codeine produces a characteristically muted opioid experience — diffuse warmth, quiet sedation, mild euphoria, and a softening of physical and emotional discomfort. The intensity is uniquely variable because codeine is a pure prodrug: the quality of the experience depends almost entirely on individual CYP2D6 metabolizer status. A poor metabolizer and an ultrarapid metabolizer taking the same dose inhabit different pharmacological realities, making genetic constitution a more powerful modifier of subjective experience than dose, set, or setting.
Compared to morphine, codeine produces a qualitatively similar but attenuated effect — milder euphoria, less sedation, and reduced respiratory depression at equivalent analgesic doses. Relative to tramadol, codeine lacks serotonergic and noradrenergic reuptake effects, producing a "cleaner" opioid profile without the stimulant-like qualities that characterize tramadol at higher doses.
Effect Grid
At common doses (60–120 mg oral), the characteristic opioid experience becomes clear: meaningful analgesia, sedation, and a pervasive sense of physical warmth and comfort. Anxiety suppression is pronounced. Cognitive euphoria manifests as a sense of well-being and contentment, while thought deceleration slows internal dialogue.
Physically, pupil constriction is obvious, constipation is reliable, and nausea may intensify — particularly in opioid-naive individuals. Itching, especially facial or nasal, is common. Respiratory rate decreases modestly. Sexual arousal suppression and appetite suppression are typical.
Functional impairment is moderate. Drowsiness may interfere with tasks requiring sustained attention. Duration is 3–5 hours of primary effects with possible residual sedation extending further.
Modeled from the effect's dose-dependence curve — not per-substance measurements.
Visual haze / noise
Pupil constriction · Tactile euphoria · Bodily heaviness · Body high · Breathing alteration · Muscle relaxation · Physical fatigue · Spontaneous body sensations · Nausea · Dizziness · Appetite suppression · Sexual arousal suppression · Dry mouth · Temperature dysregulation · Tingling / electric sensations · Excessive sweating · Nasal congestion · Vomiting · Watery eyes · Heart rate perception changes
Body high · Breathing alteration · Muscle relaxation · Physical fatigue · Spontaneous body sensations · Nausea · Dizziness · Appetite suppression · Sexual arousal suppression · Dry mouth · Tingling / electric sensations · Excessive sweating · Nasal congestion · Vomiting · Watery eyes · Heart rate perception changes
Focus suppression · Information processing suppression · Cognitive euphoria · Analysis suppression · Memory suppression · Motivation suppression · Confusion · Language suppression
Memory suppression · Motivation suppression · Confusion · Language suppression
Cognitive disconnection
Timeline
First perceptible effects — subtle warmth or mild relaxation. Onset timing reflects oral absorption and initial hepatic CYP2D6 conversion to morphine.
Analgesia, sedation, and warmth build progressively. Mild anxiolysis and mood elevation emerge. Slight pupil constriction and early gastrointestinal slowing may be noticed.
Maximum opioid effect — full analgesia, pronounced sedation, warmth, and cognitive euphoria. Respiratory rate decreases. Nausea risk is highest during this phase.
Effects gradually diminish as codeine and morphine plasma levels decline (t½ ~2. 8 hours). Analgesia fades first, followed by sedation.
Residual drowsiness, mild mood elevation, and gastrointestinal slowing may persist. Active metabolites (M6G) may extend subtle effects in extensive metabolizers.
⚠ Do not drive or operate machinery until fully alert and at least 6 hours after dosing.
Safety[edit]
- Acute toxicity
- Moderate
- Chronic toxicity
- Moderate
- Withdrawal
- Mild
- Tolerance buildup
- moderate
- Tolerance reset
- 10 days
- Physical
- Moderate
- Psychological
- Moderate
- Compulsive redosing
- Moderate
- Dose escalation
- Moderate
- Cross-tol.
- morphine · heroin · oxycodone
Toxicity
Acute Toxicity
The primary toxicity risk is respiratory depression — codeine suppresses breathing at brainstem respiratory centers through morphine, its active metabolite.[7] Forensic data place the toxicity threshold at a blood concentration of 0.3 mg/L and a potentially lethal concentration at 1.6 mg/L.[7] Most codeine-related fatalities involve other substances alongside codeine.[24]
CYP2D6-Dependent Toxicity
In CYP2D6 ultrarapid metabolizers, standard therapeutic doses of codeine can produce life-threatening respiratory depression and death due to excessive morphine conversion.
Three fatal or life-threatening pediatric cases were linked to CYP2D6 gene duplications.[5] Ultrarapid metabolizer prevalence varies by ethnicity — around 1–2% in Northern Europeans, rising to ~18% in some Latin American and African populations.[8] CYP2D6 phenotype is independently associated with adverse opioid outcomes.[25]
Neonatal Toxicity
A 2009 case attributed a neonatal death to maternal codeine use in an ultrarapid metabolizer. A subsequent critical reanalysis argued the causal link is implausible given the small quantities of opioids transferred through breast milk.[26] This case remains scientifically contested.
Organ Toxicity
Codeine is not a direct liver toxin. In codeine-acetaminophen combination products, acetaminophen — not codeine — is the hepatotoxic agent when the combination is misused.[19]
Forensic Context
Most codeine-positive fatalities involve polysubstance exposure — codeine alongside benzodiazepines, other opioids, or alcohol.[24]
Psychological Risks
Codeine does not produce psychotic episodes or persistent perceptual changes. The primary psychological risk is dependence — compulsive use patterns, particularly where codeine is available without a prescription.
OTC codeine misuse has been documented across multiple countries, often starting with legitimate pain management before escalating.[27][28][29] Shame, concealment from healthcare providers, and continued use despite harm are characteristic features.[30]
Tolerance and Dependence
Analgesic tolerance builds with repeated use through opioid downregulation.[1] Cross-tolerance occurs with all opioids acting through the same mechanism.
Physical and psychological dependence are both moderate — less intense than morphine.[1] Despite this relative mildness, codeine is the most commonly misused over-the-counter medicine globally.[30] Withdrawal produces anxiety, muscle aches, insomnia, sweating, nausea, diarrhea, and rapid heartbeat — generally milder and shorter than morphine withdrawal.
Buprenorphine/naloxone is an evidence-based treatment for codeine dependence. One published case describes successful induction 23.5 hours after the last dose, with an 18-day taper producing only mild withdrawal.[31]
Drug Interactions
The primary danger of combining codeine with other depressants is additive respiratory depression at brainstem respiratory centers, documented in forensic fatality series.[32]
CNS Depressant Combinations
Alcohol: Adding alcohol amplifies respiratory depression. Alcohol-opioid combinations are among the most common patterns in opioid-related fatalities.[32]
Benzodiazepines: Combined use carries an FDA class-wide black box warning for additive respiratory depression.
GHB/GBL: Combined depressant effects produce synergistic respiratory depression. No codeine-specific data exist; classification is based on pharmacological class inference. safety citation needed
Promethazine: The codeine-promethazine combination adds sedative and anticholinergic effects on top of opioid action. Promethazine misuse in this context is independently associated with drug-related fatalities.[13]
Serotonergic Interactions
Tramadol: Combining tramadol and codeine creates dual risk — tramadol's serotonin-raising effects can trigger a dangerous overheating and seizure response, and the two drugs also add together in respiratory depression.[33][34]
MAOIs: Opioids with serotonergic activity combined with monoamine oxidase inhibitors carry serotonin syndrome risk.[33]
Pharmacokinetic Interactions
CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion, quinidine): These block the enzyme that converts codeine to morphine, potentially causing pain suppression failure.[34] This interaction is clinically common and frequently unrecognized.
CYP3A4 inducers: May shift codeine metabolism away from morphine production, further weakening analgesic effect.
Contraindications
7 knownAbsolute Contraindications
- Children under 12 years (any indication) — FDA black box warning based on CYP2D6-related pediatric fatalities[5]
- Post-tonsillectomy/adenoidectomy (any age) — airway vulnerability combined with unpredictable metabolism creates unacceptable risk[5][35]
- Known CYP2D6 ultrarapid metabolizer status — standard doses may produce fatal respiratory depression
Relative Contraindications
- Breastfeeding: FDA advisory (disputed — critical reanalysis argues the risk is implausible[26])
- Breathing disorders or obstructive sleep apnea: Pre-existing breathing impairment amplifies respiratory depression risk
- Concurrent depressant use: Synergistic respiratory depression (see Drug Interactions)
- Liver impairment: Altered metabolism may produce unpredictable morphine levels
- Kidney impairment: accumulation of active metabolites with prolonged effect
Harm Reduction[edit]
Forms & Identification
Pharmaceutical codeine comes in tablets (often combined with acetaminophen or ibuprofen), liquid cough syrups (often combined with promethazine or guaifenesin), and less commonly as a standalone phosphate preparation. In combination products, the co-formulated ingredient often carries the greater acute risk — acetaminophen liver toxicity in tablet form, promethazine-enhanced sedation in syrups.
For codeine obtained outside a pharmacy, fentanyl test strips are the most accessible tool for detecting dangerous contaminants. Codeine from unregulated sources may be mislabeled or contain other opioids, including fentanyl.
Guidelines
- Test any codeine obtained outside a pharmacy with fentanyl test strips before use
- Start with the lowest available dose if metabolizer status is unknown — individual response to codeine varies more than for almost any other opioid
- Do not redose within four hours; codeine's peak effects may take 1–2 hours to fully develop
- Eat a light meal before dosing to reduce nausea
- Stay hydrated and keep water accessible — opioids suppress thirst awareness
- Do not use codeine to manage pain from a recent tonsillectomy or adenoidectomy at any age
- Pharmacogenomic testing for CYP2D6 status is available and directly relevant to codeine safety
Set & Setting
Codeine is a sedating opioid — the setting should accommodate drowsiness and impaired coordination. A familiar, comfortable environment with easy access to a flat resting surface is appropriate. Driving, operating machinery, and situations requiring alertness are incompatible with codeine use.
Having a sober companion present is strongly recommended, particularly for anyone who does not know their CYP2D6 metabolizer status. Ultrarapid metabolizers may experience unexpectedly strong effects from standard doses.
Aftercare & integration. Allow at least 6 hours after the last dose before driving or operating machinery. Constipation is the most persistent after-effect — maintain hydration and dietary fiber intake.
If codeine provided inadequate pain suppression, consult a healthcare provider rather than increasing the dose. Poor CYP2D6 metabolizer status may mean codeine is pharmacologically ineffective for that individual, and escalating the dose will not improve analgesia. Alternative analgesics that do not require CYP2D6 activation may be more appropriate.
Recovery & Support
Codeine dependence frequently develops gradually, particularly from over-the-counter access. If codeine use has escalated beyond its original purpose, if obtaining it has become a preoccupation, or if stopping has produced withdrawal symptoms, professional support is appropriate.
Buprenorphine/naloxone is an evidence-based treatment a healthcare provider can initiate. Tapering under medical supervision is strongly preferred over abrupt cessation. SMART Recovery and Narcotics Anonymous provide ongoing peer support.
Recovery resources, crisis lines, and support pathways → View all
Emergency Response
Recognizing an emergency: Slow or stopped breathing, blue or gray lips, unresponsiveness to voice or pain, gurgling or choking sounds, pinpoint pupils. Any of these after codeine use constitutes a medical emergency.
Immediate steps:
- 1. Call emergency services immediately
- 2. Administer naloxone if available — naloxone fully reverses opioid respiratory depression; follow the instructions on the kit
- 3. Place the person in the recovery position (on their side) if breathing but unresponsive
- 4. Stay with the person and monitor breathing continuously until help arrives
- 5. If breathing stops completely and you are trained, begin rescue breathing
- 6. Do NOT induce vomiting
What to tell the dispatcher: The substance taken (codeine, and any co-formulated ingredients such as acetaminophen or promethazine), approximate amount, time of ingestion, and current symptoms. Good Samaritan laws in many jurisdictions protect people who call for help during an overdose.
Reversal agent: Naloxone (Narcan) reverses codeine and morphine-induced respiratory depression. It is available without a prescription in many jurisdictions. A second dose may be needed — naloxone's duration of action (30–90 minutes) may be shorter than the opioid's, and respiratory depression can return.
Legal Status[edit]
Codeine is controlled internationally under the 1961 Single Convention on Narcotic Drugs (Schedule II), with scheduling varying by formulation and jurisdiction.
| Jurisdiction | Classification | Key provisions |
|---|---|---|
| United States | Schedule II (pure codeine) | Pure codeine carries highest federal scheduling. Preparations containing ≤90 mg/dosage unit are Schedule III. Cough syrups with ≤200 mg/100 mL are Schedule V[36] |
| United Kingdom | Class B (Misuse of Drugs Act 1971) | Injectable preparations classified as Class A. Low-dose preparations available as Schedule 5 pharmacy-only (OTC)[37] |
| Canada | Schedule I (CDSA) | OTC exception: preparations containing ≤8 mg codeine per tablet combined with ≥2 other medicinal ingredients[38] |
| Australia | Prescription-only (since 1 February 2018) | Previously pharmacist-only OTC. Rescheduled due to dependence and misuse concerns[6][10] |
| International | UN Single Convention Schedule II | Most signatory nations control codeine access; some permit low-dose OTC preparations under national exemptions |
Legal status varies by jurisdiction and changes over time. The trend is toward tighter restrictions on over-the-counter access.[30] This information is not legal advice — verify current status in your jurisdiction before acting on it.
Safety[edit]
- Acute toxicity
- Moderate
- Chronic toxicity
- Moderate
- Withdrawal
- Mild
- Tolerance buildup
- moderate
- Tolerance reset
- 10 days
- Physical
- Moderate
- Psychological
- Moderate
- Compulsive redosing
- Moderate
- Dose escalation
- Moderate
- Cross-tol.
- morphine · heroin · oxycodone
Toxicity
Acute Toxicity
The primary mechanism of codeine toxicity is respiratory depression, mediated by morphine acting at brainstem respiratory centers.[7] Codeine itself is a weak direct agonist; the severity of toxicity depends on how much morphine the body produces from the dose.
Forensic toxicology data place the toxicity threshold at a blood concentration of 0.3 mg/L and the potentially lethal concentration at 1.6 mg/L.[7] No human LD₅₀ has been established. In autopsy studies, morphine-to-codeine ratios were highly variable even when CYP2D6 genotype was known, indicating that genotype alone cannot reliably predict morphine levels in overdose.[7]
Codeine produces less respiratory depression than morphine at comparable analgesic doses in humans, though it is paradoxically more toxic in some animal models due to convulsant properties.[1]
CYP2D6-Dependent Toxicity
In CYP2D6 ultrarapid metabolizers, standard therapeutic doses of codeine can produce life-threatening respiratory depression and death due to excessive morphine conversion.
Three fatal or life-threatening cases in children following tonsillectomy were linked to CYP2D6 gene duplications (ultrarapid metabolizer status).[5] Ultrarapid metabolizer prevalence varies by ethnicity: approximately 1–2% of Northern Europeans, rising to ~18% in some Latin American and African populations.[8]
CYP2D6 phenotype is independently associated with adverse opioid outcomes.[25]
Neonatal Toxicity
A 2009 case attributed neonatal death to maternal codeine use with ultrarapid CYP2D6 status. Subsequent critical reanalysis argued the causal link is implausible, citing the small quantities of opioids transferred through breast milk regardless of genotype and the absence of other confirmed neonatal deaths despite widespread codeine use among breastfeeding mothers.[26] This case remains scientifically contested.
Organ Toxicity
Codeine is not a direct liver toxin. Overdose-related respiratory failure may cause secondary organ damage through oxygen deprivation.[20] Codeine-acetaminophen combinations carry the risk of acetaminophen liver toxicity if misused — the acetaminophen component, not codeine, is the hepatotoxic agent.
Forensic Context
Codeine is rarely the sole cause of death; most codeine-positive fatalities involve polysubstance exposure. Forensic case series document codeine alongside designer benzodiazepines, other opioids, and alcohol.[27]
Psychological Risks
Codeine does not produce psychotic episodes, acute psychological distress syndromes, or persistent perceptual changes. The primary psychological risk is dependence — development of compulsive use patterns, particularly where codeine is available without prescription.
OTC codeine misuse has been documented across multiple countries, frequently involving patients who initially used codeine for legitimate pain before developing dose escalation and compulsive redosing.[28][29][30] Shame, concealment from healthcare providers, and continued use despite harm are characteristic features.[31]
Tolerance and Dependence
Analgesic tolerance develops with repeated codeine use through µ-opioid receptor downregulation and related cellular adaptations.[1] Cross-tolerance occurs with all µ-opioid agonists.
Physical and psychological dependence are both moderate — less intense than morphine dependence.[1] Despite this relative mildness, codeine is the most commonly misused over-the-counter medicine globally.[31]
Withdrawal follows the opioid class pattern: anxiety, agitation, muscle aches, insomnia, sweating, runny nose, nausea, vomiting, diarrhea, and rapid heartbeat. Codeine withdrawal is generally milder and shorter than morphine withdrawal.[1]
Buprenorphine/naloxone is an effective evidence-based treatment for codeine dependence. A published case report describes successful buprenorphine induction 23.5 hours after the last codeine dose, with an 18-day gradual taper producing only mild withdrawal symptoms.[32]
Drug Interactions
The primary danger in combining codeine with other depressants is additive respiratory depression at brainstem respiratory centers, documented in forensic fatality series.[33]
CNS Depressant Combinations
Alcohol: Additive-to-synergistic respiratory depression. Alcohol-opioid combinations are among the most common patterns in opioid-related fatalities.[33]
Benzodiazepines: The FDA has issued a class-wide black box warning for concurrent opioid-benzodiazepine use due to combined respiratory depression.
GHB/GBL: Combined depressant effects produce synergistic respiratory depression. No codeine-specific data exist; classification is based on pharmacological class inference. safety citation needed
Promethazine: The codeine-promethazine combination ("purple drank") adds sedative and anticholinergic effects to opioid action. Promethazine abuse in this context is independently associated with drug-related fatalities.[13]
Serotonergic Interactions
Tramadol: Combined use creates dual risk — serotonin syndrome (from tramadol's serotonin activity) and additive respiratory depression.[34][35]
MAOIs: Serotonin syndrome risk is documented for opioids with serotonergic properties combined with monoamine oxidase inhibitors.[34]
Pharmacokinetic Interactions
CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion, quinidine): Substantially reduce codeine's conversion to morphine, potentially causing pain suppression failure.[35] This interaction is clinically common and frequently unrecognized.
CYP3A4 inducers: May shift codeine metabolism away from morphine production, further reducing analgesic effect.
Contraindications
7 knownAbsolute Contraindications
- Children under 12 years (any indication) — FDA black box warning based on CYP2D6-related pediatric fatalities[5]
- Post-tonsillectomy/adenoidectomy (any age) — airway vulnerability combined with unpredictable metabolism creates unacceptable risk[5][36]
- Known CYP2D6 ultrarapid metabolizer status — standard doses may produce fatal respiratory depression
Relative Contraindications
- Breastfeeding: FDA advisory (disputed — critical reanalysis argues the risk is implausible[26])
- Breathing disorders or obstructive sleep apnea: Pre-existing breathing impairment amplifies respiratory depression risk
- Concurrent depressant use: Synergistic respiratory depression (see Drug Interactions)
- Liver impairment: Altered metabolism may produce unpredictable morphine levels
- Kidney impairment: accumulation of active metabolites with prolonged effect
Harm Reduction[edit]
Forms & Identification
Pharmaceutical codeine circulates in several forms: tablets (often combined with acetaminophen or ibuprofen), liquid cough syrups (frequently combined with promethazine or guaifenesin), and less commonly as a standalone phosphate preparation. In combination products, the co-formulated ingredient often presents the greater acute risk — acetaminophen liver toxicity in tablet formulations, or promethazine-enhanced sedation in syrups.
Testing matters primarily for unregulated supply. Codeine obtained outside pharmaceutical channels may be mislabeled or contain other opioids, including fentanyl. Fentanyl test strips are the most accessible harm reduction tool for verifying unregulated opioid preparations.
Guidelines
- Test any codeine obtained outside a pharmacy with fentanyl test strips before use
- Start with the lowest available dose if metabolizer status is unknown — individual response to codeine varies more than for almost any other opioid
- Do not redose within four hours; codeine's peak effects may take 1–2 hours to fully develop
- Eat a light meal before dosing to reduce nausea
- Stay hydrated and keep water accessible — opioids suppress thirst awareness
- Do not use codeine to manage pain from a recent tonsillectomy or adenoidectomy at any age
- Pharmacogenomic testing for CYP2D6 status is available and directly relevant to codeine safety
Set & Setting
Codeine is a sedating opioid — the environment should accommodate drowsiness and impaired coordination. A familiar, comfortable setting with easy access to a flat resting surface is appropriate. Avoid situations requiring alertness, driving, or complex decision-making.
For those using codeine recreationally, having a sober companion present is strongly recommended, particularly for individuals who do not know their CYP2D6 metabolizer status. Ultrarapid metabolizers may experience unexpectedly strong effects from standard doses.
Aftercare & integration. After codeine use, allow adequate time for the drug to clear before driving or operating machinery — a minimum of 6 hours after the last dose. Constipation is the most persistent after-effect; maintain hydration and dietary fiber intake.
If codeine was used for pain management and provided inadequate relief, consult a healthcare provider rather than escalating the dose. Poor CYP2D6 metabolizer status may mean codeine is pharmacologically ineffective for a given individual, and dose escalation will not improve analgesia but will increase side effect burden. Alternative analgesics that do not require CYP2D6 activation may be more appropriate.
Recovery & Support
Codeine dependence frequently develops insidiously, particularly from over-the-counter access. If codeine use has escalated beyond its original purpose, if obtaining codeine has become a preoccupation, or if attempts to stop have produced withdrawal symptoms, professional support is appropriate.
Buprenorphine/naloxone is an effective evidence-based treatment for codeine dependence and can be initiated by a healthcare provider. Tapering under medical supervision is strongly preferred over abrupt cessation. Peer support organizations such as SMART Recovery and Narcotics Anonymous provide ongoing community support.
Recovery resources, crisis lines, and support pathways → View all
Emergency Response
Recognizing an emergency: Slow or stopped breathing, blue or gray lips, unresponsiveness to voice or pain, gurgling or choking sounds, pinpoint pupils. Any of these after codeine use constitutes a medical emergency.
Immediate steps:
- 1. Call emergency services immediately
- 2. Administer naloxone if available — naloxone fully reverses opioid respiratory depression; follow the instructions on the kit
- 3. Place the person in the recovery position (on their side) if breathing but unresponsive
- 4. Stay with the person and monitor breathing continuously until help arrives
- 5. If breathing stops completely and you are trained, begin rescue breathing
- 6. Do NOT induce vomiting
What to tell the dispatcher: The substance taken (codeine, and any co-formulated ingredients such as acetaminophen or promethazine), approximate amount, time of ingestion, and current symptoms. Good Samaritan laws in many jurisdictions protect people who call for help during an overdose.
Reversal agent: Naloxone (Narcan) is an opioid antagonist that reverses codeine/morphine-induced respiratory depression. It is available without a prescription in many jurisdictions. A second dose may be needed — naloxone's duration of action (30–90 minutes) may be shorter than the opioid's, and respiratory depression can return.
Legal Status[edit]
Codeine is controlled internationally under the 1961 Single Convention on Narcotic Drugs (Schedule II). Most jurisdictions classify codeine as a controlled substance, though the scheduling tier varies based on formulation concentration and combination with other active ingredients.
| Jurisdiction | Classification | Key provisions |
|---|---|---|
| United States | Schedule II (pure codeine) | Pure codeine carries highest federal scheduling. Preparations containing ≤90 mg/dosage unit are Schedule III. Cough syrups with ≤200 mg/100 mL are Schedule V[37] |
| United Kingdom | Class B (Misuse of Drugs Act 1971) | Injectable preparations classified as Class A. Low-dose preparations available as Schedule 5 pharmacy-only (OTC)[38] |
| Canada | Schedule I (CDSA) | OTC exception: preparations containing ≤8 mg codeine per tablet combined with ≥2 other medicinal ingredients[39] |
| Australia | Prescription-only (since 1 February 2018) | Previously pharmacist-only OTC. Rescheduled due to dependence and misuse concerns[6][10] |
| International | UN Single Convention Schedule II | Most signatory nations control codeine access; some permit low-dose OTC preparations under national exemptions |
The trend across jurisdictions is toward increasing restriction of OTC codeine access. Australia's 2018 rescheduling represents the most significant recent regulatory shift, driven by evidence of widespread OTC codeine misuse and dependence.[31]
Frequently asked questions
What is Codeine?
Codeine is a naturally occurring morphinan opioid and depressant that acts as a prodrug, requiring hepatic CYP2D6 conversion to morphine. Isolated from opium in 1832 by Pierre-Jean Robiquet, it remains one of the most widely prescribed analgesics and antitussives globally.
How long does Codeine last?
Total duration by route of administration — Oral: 3 – 6 hours. Ranges vary with dose and individual factors; see the duration table for onset, peak, and after-effect phases.
Where can I find dosage information for Codeine?
Psychedex publishes a dosage table for Codeine on this page covering these routes of administration: oral. Dose values are presented only in that table, always alongside their route of administration.
What are dangerous combinations with Codeine?
The highest-severity documented interactions for Codeine: Benzodiazepines, Barbiturates (Lethal), GHB, Baclofen (Lethal), GHB, GBL (Lethal), Local anesthetics (Lethal), 5-HTP, Tryptophan (Dangerous). See the interactions section for the full list with mechanisms and severity levels.
Is Codeine addictive?
Psychedex records tolerance and dependence data rather than a yes/no rating. Recorded tolerance indicators for Codeine: tolerance builds moderate; full reset takes 10 days; cross-tolerance develops with morphine, heroin, oxycodone, hydrocodone, fentanyl, tramadol, dihydrocodeine. See the article's dependence and safety sections for recorded dependence data.
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references
- [1]
- [2] ^abMattia C, Coluzzi F (2015) A look inside the association codeine-paracetamol: clinical pharmacology supports analgesic efficacy — European Review for Medical and Pharmacological Sciences PMID:25720726
- [3] ^Volpe DA, McMahon Tobin GA, Mellon RD, et al. (2011) Uniform assessment and ranking of opioid mu receptor binding constants for selected opioid drugs — Regulatory Toxicology and Pharmacology doi:10.1016/j.yrtph.2010.12.007
- [4]
- [5]
- [6] ^abcSchaffer AL, Cairns R, Brown JA, et al. (2020) Changes in sales of analgesics to pharmacies after codeine was rescheduled as a prescription only medicine — Medical Journal of Australia doi:10.5694/mja2.50552
references
- [1]
- [2] ^abcMattia C, Coluzzi F (2015) A look inside the association codeine-paracetamol: clinical pharmacology supports analgesic efficacy — European Review for Medical and Pharmacological Sciences PMID:25720726
- [3] ^Volpe DA, McMahon Tobin GA, Mellon RD, et al. (2011) Uniform assessment and ranking of opioid mu receptor binding constants for selected opioid drugs — Regulatory Toxicology and Pharmacology doi:10.1016/j.yrtph.2010.12.007
- [4]
- [5]
- [6] ^abcSchaffer AL, Cairns R, Brown JA, et al. (2020) Changes in sales of analgesics to pharmacies after codeine was rescheduled as a prescription only medicine — Medical Journal of Australia doi:10.5694/mja2.50552