O-Desmethyltramadol
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O-DSMT (O-desmethyltramadol, desmetramadol) is a synthetic opioid of the aminocyclohexanol class. It is the principal active metabolite of tramadol, carrying roughly 100-fold greater µ-opioid receptor affinity than the parent compound.[1] O-DSMT is a full µ-opioid receptor agonist, producing its effects by suppressing pain signaling and modulating monoamine reuptake.
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O-desmethyltramadol was identified in the early 1990s as the metabolite responsible for tramadol's analgesic activity, decades after tramadol's synthesis by Grünenthal GmbH in 1962. It entered public health discourse after nine fatalities in Sweden (2009–2010) linked to "Krypton," a kratom product adulterated with the compound.[2] It is now a Class B substance in the United Kingdom and circulates as a research chemical in jurisdictions where it remains unscheduled.
Subjective effects include pain suppression, sedation, cognitive euphoria, and anxiolysis. The experience is a warm, muted opioid blanket — quieter and more purely sedating than tramadol, which retains noradrenergic and serotonergic coloring from the parent compound. Nausea, constipation, and cognitive slowing are characteristic unsought effects.
O-DSMT produces physical dependence with repeated use, and respiratory depression is the established mechanism of fatal overdose.[2] Withdrawal is atypical among opioids, combining classical symptoms with serotonergic features including anxiety and sensory disturbance.[3] No controlled studies have characterized standalone O-DSMT's subjective effects or toxicity — current understanding rests on tramadol pharmacology, Phase 1 clinical data,[4] and forensic case reports.
History[edit]
Discovery
Tramadol was synthesized in 1962 by Grünenthal GmbH in Germany and launched commercially in 1977.[5] O-desmethyltramadol was identified in the early 1990s as tramadol's principal active metabolite, when studies demonstrated that tramadol's analgesic activity depends on CYP2D6-mediated O-demethylation.[1][6] The (+)-enantiomer was found to carry the opioid activity, while the (-)-enantiomer mediates monoamine reuptake inhibitor — a division of labor that defined the pharmacology of tramadol-class compounds.
Clinical and public health significance
ODT appeared as a standalone research chemical for online purchase in the late 2000s. Between 2009 and 2010, nine young people in Sweden died after consuming "Krypton," a kratom-based product containing undeclared ODT.[2][7] Forensic analysis confirmed ODT as a direct additive rather than a tramadol metabolite.[2] Syntrix Biosystems subsequently advanced ODT as a clinical analgesic under the name desmetramadol, completing Phase 1 trials that demonstrated dose-equivalence with tramadol and CYP2D6-independent efficacy.[4]
Regulation and present status
The Swedish fatalities triggered regulatory response across Europe. The United Kingdom classified ODT as a Class B substance in February 2013 following recommendation by the Advisory Council on the Misuse of Drugs.[8] Tramadol was placed into Schedule IV in the United States in 2014,[9] though ODT itself remains without specific federal scheduling. Its identification as a G-protein biased µ-opioid agonist[10] has sustained pharmacological interest, but no Phase 2 or Phase 3 clinical results have been published.
The abbreviation "O-DSMT" is the dominant identifier in research chemical markets, while "desmetramadol" (INN) and "Omnitram" (brand name) belong to the clinical development program. The name "Krypton" — the herbal blend responsible for the Swedish deaths — became synonymous in harm-reduction circles with the danger of undeclared synthetic opioids in marketed "natural" products, predating analogous concerns about fentanyl-adulterated supplies by several years.
Chemistry[edit]
Structural class
O-Desmethyltramadol belongs to the aminocyclohexanol family, sharing its core scaffold with tramadol, tapentadol, and venlafaxine. The compound is a 2-aminomethylcyclohexan-1-ol bearing a 3-hydroxyphenyl group — the product of removing the methyl group from tramadol's 3-methoxy position to expose a free phenol. This single structural change — methyl ether to phenol — is responsible for the dramatic increase in µ-opioid receptor affinity.[11]
Molecular properties
| Molecular formula | C₁₅H₂₃NO₂ |
| Molecular weight | 249.35 g/mol |
The structure consists of a cyclohexane ring with a tertiary alcohol at C-1, a dimethylaminomethyl group at C-2, and a 3-hydroxyphenyl substituent. The free phenol is the key pharmacophore for opioid receptor binding. The dimethylamino group makes the compound basic, allowing it to form salts readily under acidic conditions.
Stereochemistry
Two chirality centers at C-1 and C-2 produce four possible stereoisomers in two enantiomeric pairs (trans and cis). The pharmacologically active isomer is (1R,2R)-(+)-trans-O-desmethyltramadol, which carries the strongest µ-opioid receptor binding.[1] The (1S,2S)-(-)-trans enantiomer has substantially lower opioid affinity but contributes serotonin and norepinephrine reuptake inhibitor. Racemic ODT, as typically encountered on the research chemical market, contains both enantiomers and produces a mixed pharmacological profile.[5]
Physical properties
Published data on the free base are limited. The hydrochloride salt is a white to off-white crystalline solid. Solubility data for standalone ODT have not been published, though the compound's basic amine and phenol groups suggest moderate to high aqueous solubility under acidic conditions. Stability and degradation pathways remain uncharacterized in the literature.citation needed
Pharmacology[edit]
Mechanism of action
O-Desmethyltramadol acts through two mechanisms distributed across its enantiomers. The (+)-enantiomer is a full agonist at the µ-opioid receptor, activating inhibitory G-protein signaling that reduces neuronal excitability — the same fundamental mechanism behind morphine, oxycodone, and other classical opioids.[12] This produces analgesia, sedation, euphoria, and respiratory depression.
At the spinal cord level, ODT suppresses excitatory nerve transmission while enhancing inhibitory signaling in pain-processing neurons.[13][14] In rats, ODT at 5 mg/kg intravenous matched the spinal effects of tramadol at 15 mg/kg, consistent with a roughly 3:1 potency ratio.[14] All spinal effects were blocked by naloxone, confirming dependence on opioid receptors.[14]
The (-)-enantiomer inhibits serotonin and norepinephrine reuptake, contributing to pain suppression through descending inhibitory pathways and conferring serotonin syndrome risk.[15][6] This monoaminergic activity is weaker than in the parent compound tramadol. ODT also suppresses TRPA1 ion channels without affecting TRPV1, providing an additional non-opioid analgesic mechanism.[16]
Pharmacokinetics
When generated from tramadol, ODT production depends on liver enzyme CYP2D6.[17][18] Individuals who metabolize drugs slowly through this enzyme produce less ODT and experience reduced analgesia; ultrarapid metabolizers produce dangerously elevated levels.
When administered directly as desmetramadol, this metabolic variability is bypassed entirely — a clinically significant advantage. Phase 1 trials established that 20 mg desmetramadol every 6 hours produces equivalent plasma levels to 50 mg tramadol every 6 hours,[4] and that CYP2D6 inhibition does not affect desmetramadol's efficacy.[4]
The half-life is approximately 7.5 hours in young adults, increasing by roughly 50% in elderly subjects.[19][20]
Neuroscience[edit]
Where it acts
ODT's µ-opioid receptor activation engages the mesolimbic dopamine system — the brain's reward circuitry. Opioid receptor activation in the ventral tegmental area releases dopamine in the nucleus accumbens, producing the warmth, contentment, and reinforcement characteristic of the opioid experience. Tramadol-dependent subjects show euphoria scores comparable to heroin and methamphetamine users on standardized measures,[21] confirming robust mesolimbic pathway activation.
In pain-processing circuits, ODT engages descending modulatory pathways from the brainstem to the spinal cord, suppressing excitatory transmission and enhancing inhibitory signaling at spinal dorsal horn neurons.[13][14] The (-)-enantiomer augments this through serotonergic and noradrenergic pathways independent of opioid receptors.[15]
Why it matters
ODT belongs to a class of "biased agonists" — compounds that preferentially activate one intracellular signaling pathway over another at the µ-opioid receptor. ODT favors G-protein signaling over β-arrestin2 recruitment,[10] a profile shared with oliceridine. This bias may partially explain why tramadol-class compounds produce less respiratory depression per unit of analgesia than classical opioids, though this advantage disappears at overdose levels.
The mesolimbic reward circuitry also explains compulsive redosing: repeated exposure sensitizes "wanting" circuits even as tolerance diminishes subjective pleasure — a dissociation between craving and reward that drives dose escalation.
Whether ODT's G-protein bias translates to slower tolerance development in humans remains untested — the biased agonism framework predicts this, but no human tolerance kinetics studies exist. The absolute oral bioavailability of standalone desmetramadol has not been published despite completed clinical trials. How ODT's subjective profile differs from tramadol's under controlled conditions — predicted to be more purely opioid with less serotonergic coloring — has never been formally compared.
Subjective Effects[edit]
O-Desmethyltramadol delivers a predominantly opioid experience — warmth spreading from the core, pain suppression, physical and mental sedation, and a diffuse contentment that dampens both physical discomfort and emotional distress. At lower doses the effect is subtle: a gentle anxiolysis and mild physical comfort. At higher doses, sedation deepens into a heavy, enveloping calm with pronounced tactile pleasure and reduced motivation to move or engage. Set and setting modulate intensity but not character — the experience is recognizably opioid regardless of context.
Compared to tramadol, standalone ODT produces a cleaner opioid signature with less of the noradrenergic alertness and serotonergic warmth that tramadol users report. Relative to classical opioids like morphine or oxycodone, ODT occupies a lower-potency position with a gentler ceiling — though at high doses the distinction narrows and all the risks of full µ-agonism apply.
Effect Grid
Clear opioid effects dominate — pronounced pain suppression, definite sedation, and distinct cognitive euphoria with anxiolysis. Tactile euphoria and body high are well-established: a warm, enveloping heaviness that many describe as deeply comfortable. Emotional suppression flattens negative affect. Thought deceleration and focus suppression become noticeable.
Constipation and nausea are likely. Pupil constriction is evident. Mild respiratory depression is possible, particularly toward the upper end. Dry mouth, dizziness, and motor control impairment may occur. Sexual arousal suppression is common.
This range roughly corresponds to the clinically tested dose (20 mg desmetramadol ≈ 50 mg tramadol). Functional impairment is significant — driving and complex tasks are unsafe. Duration is 6–10 hours orally.
Modeled from the effect's dose-dependence curve — not per-substance measurements.
Constipation · Body high · Spontaneous body sensations · Nausea · Appetite suppression · Tactile euphoria · Bodily heaviness · Muscle relaxation · Physical fatigue · Sexual arousal suppression · Dizziness · Excessive sweating · Dry mouth · Vomiting · Yawning · Respiratory depression
Nausea · Appetite suppression · Tactile euphoria · Bodily heaviness · Muscle relaxation · Physical fatigue · Sexual arousal suppression · Dizziness · Excessive sweating · Dry mouth · Vomiting · Yawning
Confusion
Timeline
First perceptible warmth and subtle mood shift emerge. A gentle awareness of opioid activity begins without clear euphoria or sedation.
Opioid warmth intensifies steadily. Analgesia builds, mild sedation and anxiolysis become noticeable, and body high begins to develop.
Full opioid effects — maximal analgesia, sedation, cognitive euphoria, and tactile euphoria. Pupil constriction and respiratory depression risk are at their highest.
Gradual diminution of effects. Residual warmth and sedation persist while euphoria and analgesia progressively fade. Cognitive function slowly returns.
Mild residual sedation and possible drowsiness. Faint warmth may linger.
⚠ Do not drive or operate heavy machinery for at least 10 hours after dosing — residual sedation and cognitive impairment persist beyond perceived offset.
Safety[edit]
- Acute toxicity
- High
- Chronic toxicity
- High
- Withdrawal
- Moderate
- Tolerance buildup
- moderate
- Tolerance reset
- 10.5 days
- Physical
- Moderate
- Psychological
- Moderate
- Compulsive redosing
- Moderate
- Dose escalation
- Moderate
- Cross-tol.
- morphine · oxycodone · fentanyl
Toxicity
Acute toxicity
No formal lethal dose has been established for O-desmethyltramadol in any species.safety citation needed
Respiratory depression is the primary mechanism of death in ODT overdose. Fatal outcomes have been documented at postmortem blood concentrations of 0.4–4.3 µg/g, in all cases alongside mitragynine co-administration.[2] No confirmed lethal dose for ODT alone has been established. Naloxone reverses ODT-induced respiratory depression but may require repeated doses, as ODT's half-life (~7.5 hours) far exceeds naloxone's (~1 hour).[14]
For context, tramadol-attributed deaths consistently involved co-intoxicants,[22] suggesting a wider margin than classical opioids when used alone — though fatal single-substance tramadol poisonings have been documented at high blood concentrations.[23] A confirmed suicidal tramadol overdose produced femoral blood levels of 61.83 µg/mL with ODT detected in all tissues.[24]
Seizure risk
Tramadol lowers the seizure threshold through serotonergic mechanisms independent of opioid activity.[25] ODT demonstrates a biphasic pattern in animal models: anticonvulsant at analgesic doses, proconvulsant at supratherapeutic doses.[26] Tramadol-associated seizures are a significant clinical problem in regions of high misuse.[27]
Benzodiazepines abolish tramadol-induced seizures but significantly worsen respiratory depression[25] — this apparent seizure protection masks increased mortality risk.
Serotonin syndrome
The serotonin reuptake inhibitor carried by the (-)-enantiomer creates a serotonin syndrome risk absent from classical opioids.[28][15] Fatal serotonin syndrome has occurred with the combination of tramadol, an monoamine oxidase inhibitor (moclobemide), and clomipramine.[29]
Pediatric vulnerability
Children demonstrate dose-related central nervous system depression, respiratory depression, and seizures with tramadol/ODT exposure.[30] CYP2D6 ultrarapid metabolizers produce elevated ODT levels from standard tramadol doses, placing this group at disproportionate risk.
Psychological risks
The psychological risk profile has not been characterized independently of tramadol.
Tramadol-dependent subjects demonstrate euphoria comparable to heroin and methamphetamine users on standardized measures,[21] indicating significant abuse potential. Standalone ODT, with higher opioid receptor affinity and more direct opioid effects, may carry equal or greater euphoric potential.
µ-Opioid-mediated reward creates compulsive redosing pressure, particularly in individuals with prior opioid exposure or substance use disorders. Opioid-induced cognitive impairment — attention deficits, slowed reactions, impaired judgment — is expected at common doses.
Abrupt cessation after repeated use produces opioid withdrawal (restlessness, muscle pain, insomnia, dysphoria) with an additional atypical component: anxiety, confusion, and sensory disturbance reflecting serotonergic withdrawal.[31]
Tolerance & dependence
Tolerance develops through standard opioid mechanisms — receptor desensitization and downregulation with repeated use.[32] Cross-tolerance is complete with all µ-opioid agonists including morphine, oxycodone, fentanyl, and heroin.[32]
ODT's G-protein biased signaling[10] may theoretically slow tolerance development, but no ODT-specific tolerance studies have been conducted in humans. Community-sourced claims of a 5-day tolerance half-life lack published support.citation needed
Physical dependence has been established in a controlled human study.[3] Withdrawal is atypical among opioids: classical symptoms (restlessness, muscle pain, tearing, diarrhea) combine with serotonergic and noradrenergic features (anxiety, paranoia, confusion, sensory abnormalities). A case report of cessation after more than 10 years of daily tramadol use documented atypical symptoms including excessive mucus production.[31]
Tramadol/ODT has been used in opioid withdrawal management with efficacy comparable to methadone,[33][34] demonstrating cross-tolerance with full agonists.
Drug interactions
Serotonergic interactions
The (-)-enantiomer's serotonin reuptake inhibition creates a risk of serotonin syndrome when ODT is combined with other serotonergic agents.[28][15] MAO inhibitors are an absolute contraindication — fatal serotonin syndrome has occurred with this combination.[29] SSRIs, SNRIs, tricyclic antidepressants, linezolid, and triptans all carry elevated risk.
Central nervous system depressants
Combination with alcohol, benzodiazepines, GHB/GBL, ketamine, or other opioids produces additive to synergistic respiratory depression. Alcohol is the most commonly co-detected substance in tramadol overdose deaths.[22] The Swedish Krypton deaths demonstrate the lethality of combining ODT with mitragynine even at moderate individual doses.[2]
Metabolic interactions
For tramadol-derived ODT: CYP2D6 inhibitors (paroxetine, fluoxetine, bupropion) reduce ODT production and impair analgesia. CYP3A4 inducers (rifampicin) decrease exposure to both tramadol and ODT.[17] For standalone ODT: CYP2D6 inhibitors have no effect on pharmacokinetics.[4]
Drug testing
ODT produces false-positive phencyclidine (PCP) results on EMIT II+ immunoassay.[35] It is not detected by standard opioid screens calibrated for morphine or codeine. Confirmatory testing by mass spectrometry is required for accurate identification.
Contraindications
Absolute contraindications
- Concurrent MAO inhibitor use — fatal serotonin syndrome risk[29]
- Severe respiratory insufficiency — µ-opioid agonism directly depresses breathing
- Acute intoxication with other depressants — additive respiratory depression with alcohol, benzodiazepines, other opioids, or sedative-hypnotics
Relative contraindications
- Concurrent serotonergic medications (SSRIs, SNRIs, tricyclics, triptans, linezolid) — elevated serotonin syndrome risk[28]
- Seizure disorders or lowered seizure threshold — proconvulsant effects at supratherapeutic doses[26]
- CYP2D6 ultrarapid metabolizer status (when ODT is generated from tramadol) — dangerously elevated production[32]
- Pediatric patients — enhanced CNS depression and seizure risk[30]
- Elderly patients — approximately 50% longer elimination half-life[19]
- Kidney impairment — prolonged elimination[12]
- Pregnancy — risk of neonatal opioid withdrawal syndromesafety citation needed
Harm Reduction[edit]
Forms & Identification
O-DSMT is most commonly encountered as a white to off-white crystalline powder (hydrochloride salt), but has also appeared in pressed tablets, pellets, and capsules. The compound cannot be identified by appearance alone — it resembles dozens of other white powders sold as research chemicals or used as adulterants.
Standard opioid immunoassays calibrated for morphine and codeine will not detect O-DSMT. False positives for PCP on EMIT II+ assays have been documented,[35] which may cause confusion during emergency treatment. Fentanyl test strips do not detect O-DSMT. Given the absence of reliable field identification, laboratory analysis or a drug checking service offers the only meaningful purity verification.
Guidelines
- Test every batch — visual inspection cannot verify identity or purity
- Use a milligram-precision scale (0.001 g) — O-DSMT is active at low milligram doses and eyeballing is dangerously imprecise
- Start with the lowest plausible dose and wait at least 90 minutes before considering redosing — oral onset is gradual and peak effects are delayed
- Do not use alone — have a sober companion present who knows how to administer naloxone and call emergency services
- Keep naloxone readily accessible — multiple doses may be needed given O-DSMT's long half-life relative to naloxone's
- Do not combine with alcohol, benzodiazepines, other opioids, or any sedating substance — respiratory depression is additive to synergistic
- Avoid redosing during the same session — compulsive redosing is a recognized pattern with µ-opioid agonists and each additional dose compounds respiratory risk
- Eat lightly beforehand and stay hydrated — nausea and constipation are common
Set & Setting
O-DSMT is a sedating opioid that impairs coordination, judgment, and respiratory drive. Use in environments where falling asleep unmonitored is possible carries elevated risk — the compound's long duration means respiratory depression can develop hours into the experience. A sober companion who understands opioid overdose signs and has access to naloxone materially reduces fatality risk.
Avoid driving, swimming, or operating machinery. Cold environments increase risk: opioid sedation suppresses shivering and awareness of hypothermia.
Aftercare & integration. Physical recovery from O-DSMT use is straightforward at moderate doses: hydration, light meals, and rest. Constipation may persist for 1–2 days; adequate fluid and fiber intake helps.
The greater aftercare concern is psychological. Opioids produce a reliable, complete suppression of distress that can create powerful associations between the substance and emotional relief. Honest self-assessment after use — whether the motivation was curiosity, pain management, or emotional escape — helps identify patterns before they become habitual. If use is recurring and motivated by emotional relief rather than pain, professional consultation is warranted.
Recovery & Support
O-DSMT dependence follows opioid patterns: escalating doses, use despite consequences, and withdrawal-driven continued use. Withdrawal from tramadol-class compounds is atypical — it combines standard opioid symptoms (muscle pain, restlessness, diarrhea, insomnia) with serotonergic features (anxiety, confusion, sensory disturbance),[31] making it less predictable than classical opioid withdrawal.
Medical supervision for withdrawal is strongly recommended. Tapering under guidance is safer than abrupt cessation. Medication-assisted treatment options developed for opioid use disorder — including buprenorphine and methadone — are effective for tramadol-class dependence. SAMHSA's National Helpline (1-800-662-4357) provides free, confidential treatment referrals. Peer support through SMART Recovery or Narcotics Anonymous offers ongoing structure for sustained recovery.
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Emergency Response
Recognizing an emergency: slow, shallow, or stopped breathing; blue or grey lips and fingertips; gurgling or snoring sounds from an unconscious person; unresponsiveness to shouting or sternal rub; pinpoint pupils. These signs distinguish opioid overdose from heavy sedation.
Immediate steps:
- 1. Call emergency services immediately
- 2. Administer naloxone if available — intranasal or intramuscular, following the product's instructions
- 3. Place the person in the recovery position (on their side) to prevent choking
- 4. If breathing stops entirely, begin rescue breathing or CPR
- 5. Stay with the person — O-DSMT outlasts naloxone, and breathing may stop again after naloxone wears off; redose naloxone every 2–3 minutes if breathing does not resume
- 6. Do NOT induce vomiting in an unconscious person
What to tell the dispatcher: the substance taken (O-desmethyltramadol / O-DSMT, a synthetic opioid), estimated amount and time of ingestion, current symptoms, and whether naloxone has been administered. Good Samaritan laws in many jurisdictions protect people who call for help during an overdose from drug-related prosecution.
Reversal agent: Naloxone (Narcan) reverses opioid-induced respiratory depression. Multiple doses may be required — O-DSMT's half-life (~7.5 hours) far exceeds naloxone's (~1 hour). Monitor breathing continuously for at least 4 hours after the last naloxone dose.
Legal Status[edit]
O-Desmethyltramadol occupies an unusual regulatory position: specifically controlled in some jurisdictions, covered by parent-compound scheduling or structural analog legislation in others, and legally ambiguous where only tramadol — not its metabolites — is named in controlled substance schedules.
| Jurisdiction | Classification | Key provisions |
|---|---|---|
| United Kingdom | Class B (Misuse of Drugs Act 1971); Schedule 1 (2001 Regulations) | Controlled since February 26, 2013, following ACMD recommendation prompted by the Swedish Krypton deaths. No recognized legitimate medical use. Also covered by the Psychoactive Substances Act 2016.[8] |
| United States | Not separately federally scheduled | Tramadol is DEA Schedule IV (effective August 18, 2014).[9] ODT is not individually listed. The Federal Analogue Act applies only to analogs of Schedule I/II substances; tramadol's Schedule IV placement creates legal ambiguity. State analogue laws may apply. |
| Germany | Not controlled under BtMG | Not listed in the Betäubungsmittelgesetz. Sale for human consumption may be restricted under the German Medicines Act (§2 AMG).citation needed |
| Sweden | Controlled | Scheduled following the 2009–2010 Krypton fatalities.[7] |
| Australia | Unverified | Tramadol is Schedule 8 (controlled drug). ODT-specific scheduling not confirmed in public sources.citation needed |
| Canada | Unverified | Tramadol is controlled under the Controlled Drugs and Substances Act. ODT-specific status not confirmed.citation needed |
Analogue legislation
The applicability of analogue legislation to ODT varies by jurisdiction. In the United States, the Federal Analogue Act treats substances "substantially similar" to Schedule I or II compounds as equivalent when intended for human consumption — but tramadol is Schedule IV, placing ODT outside this framework's explicit scope. In the United Kingdom, explicit Class B listing eliminates ambiguity. In jurisdictions without specific ODT scheduling, enforcement typically relies on medicines legislation rather than narcotics law.
references
- [1] ^abcLai J, Ma SW, Porreca F, Raffa RB (1996) Tramadol, M1 metabolite and enantiomer affinities for cloned human opioid receptors expressed in transfected HN9.10 neuroblastoma cells — European Journal of Pharmacology PMID:8982709
- [2]
- [3] ^abLanier RK, Lofwall MR, Mintzer MZ, Bigelow GE, Strain EC (2010) Physical dependence potential of daily tramadol dosing in humans — Psychopharmacology doi:10.1007/s00213-010-1919-3
- [4] ^abcdeZebala JA, Searle SL, Webster LR, Johnson MS, Schuler AD, Maeda DY, Kahn SJ (2019) Desmetramadol Has the Safety and Analgesic Profile of Tramadol Without Its Metabolic Liabilities: Consecutive Randomized, Double-Blind, Placebo- and Active Comparator-Controlled Trials — The Journal of Pain doi:10.1016/j.jpain.2019.04.005
- [5] ^abAgranat I, D'Acquarica I (2024) Chiral Switches of Tramadol Hydrochloride, a Potential Psychedelic Drug-Past and Future — ACS Medicinal Chemistry Letters doi:10.1021/acsmedchemlett.4c00322
- [6] ^abFrink MC, Hennies HH, Englberger W, Haurand M, Wilffert B (1996) Influence of tramadol on neurotransmitter systems of the rat brain. — Arzneimittel-Forschung PMID:8955860