Zopiclone

Discovery

Zopiclone was synthesized by Rhône-Poulenc in the early 1970s as part of a program to develop sleeping pills structurally distinct from benzodiazepines.^[1] It was patented in 1972 as the first of its chemical class — a scaffold unrelated to barbiturates, benzodiazepines, or imidazopyridines. First human studies, published in 1982–1983,^[6][7] established its core absorption and elimination parameters.

Clinical adoption

France approved zopiclone in 1986, with Canada, Australia, the United Kingdom, and continental Europe following.^[1] It was marketed as superior to benzodiazepines for preserving deep sleep with less next-day grogginess. Sepracor later isolated its active form — eszopiclone — which the US FDA approved in December 2004 as the first hypnotic approved without a time limit on use.^[2]

Scheduling

The US Drug Enforcement Administration placed zopiclone in Schedule IV in April 2005.^[8] In the United Kingdom it is classified under Schedule 4, Part I of the Misuse of Drugs Act 1971; Canada lists it under Schedule IV of the Controlled Drugs and Substances Act. New Zealand reclassified it as a Class C5 controlled drug in 2022.citation needed

Structural class

Zopiclone is the prototypical cyclopyrrolone — a chemical family structurally unrelated to benzodiazepines, barbiturates, or zolpidem's imidazopyridine class.^[1] Its molecular formula is C₁₇H₁₇ClN₆O₃ (molecular weight 388.8 g/mol). The primary site the body breaks it down is a carboxylate ester linkage that the liver cleaves as the dominant metabolic route, accounting for more than half the administered dose.^[7]

Stereochemistry

Zopiclone exists as a racemic mixture — two mirror-image forms in equal proportion.^[9] Only the S-(+)-enantiomer produces the hypnotic effect; the R-(-)-enantiomer is inactive. The isolated active form is marketed separately as eszopiclone (Lunesta).

Mechanism of action

Zopiclone amplifies GABA — the brain's primary calming signal — by binding at the benzodiazepine site on GABA-A receptors.^[1] It does not activate the receptor directly; it makes the brain's own GABA more effective, increasing the frequency of inhibitory signaling. Unlike zolpidem, zopiclone engages multiple receptor subtypes, which explains its combined sedative, anxiolytic, and muscle-relaxing effects.^[10][11]

Pharmacokinetics

Zopiclone is rapidly absorbed after oral dosing, reaching peak blood levels within 30–60 minutes.^[9] Oral bioavailability is approximately 77%, with about 20–25% lost to first-pass liver metabolism.^[12] The liver processes it primarily through the enzyme CYP3A4,^[13][14] and less than 7% exits unchanged in urine.^[7] The characteristic bitter taste appears in saliva around 24 minutes after dosing,^[3] and the compound passes into breast milk at concentrations approaching those in blood.^[7]

Where it acts

Zopiclone promotes sleep by suppressing the brain's wake-promoting centers — neurons that use histamine, norepinephrine, and serotonin to keep the body alert — while allowing sleep-promoting neurons in the hypothalamus to take over.^[1][11] This produces a relatively fast shift from wakefulness into sleep, with better preservation of deep sleep than most benzodiazepines.

Residual impairment — slowed reactions, memory gaps, impaired driving performance — persists well beyond subjective sedation, lasting up to 10 hours after a standard dose.^[4][15] A person may feel alert and functional while meaningfully impaired.

Why it matters

The memory-impairing effects arise from disruption of memory encoding in the hippocampus — the brain region that converts experiences into lasting memories.^[5] GABA amplification blocks the synaptic changes this process depends on. This explains both the therapeutic forgetting of pre-sleep anxiety and the concerning amnesia for activities performed during incomplete arousal.

Toxicity

Fatality risk rises sharply with polydrug overdose. Post-mortem data include fatalities with zopiclone at toxic blood levels alongside other substances,^[17][18] and European surveillance data found zopiclone most frequently involved in overdose events among the three Z-drugs.^[19]

Psychological risks

Zopiclone can cause memory gaps for events after dosing, complex sleep behaviors — driving, eating, or walking during partial arousal with no memory afterward — visual hallucinations, and behavioral disinhibition.^[5][22] Complex sleep behaviors are elevated by concurrent sedating medications and pre-existing sleep disorders.^[23][24]

Irritability, emotional lability, and paradoxical agitation have been reported, with higher incidence in elderly patients.^[5][25] Epidemiological data associate long-term hypnotic use with increased depression and mortality risk, though whether this is causal remains unestablished.^[26]

Tolerance and dependence

Tolerance to the sleep-promoting effects develops more slowly than for benzodiazepines. A 17-week sleep laboratory study showed sustained efficacy without measurable tolerance,^[27] and eszopiclone data extend to 12 months.^[2] Cross-tolerance with benzodiazepines is expected given the shared binding site.^[1][11]

Physical and psychological dependence develop with prolonged use. Dependence risk is equivalent to zolpidem^[19] and is consistently characterized as lower than benzodiazepines but clinically significant — particularly in people with a history of substance use disorder.^[1][14] Stopping after chronic use brings rebound effect insomnia, anxiety, disturbed dreams, and nausea;^[2] gradual dose tapering is recommended.^[28]

Drug interactions

Zopiclone is processed by the liver enzyme CYP3A4. Drugs that block this enzyme raise zopiclone blood levels and intensify sedation: erythromycin,^[9][13][14] azole antifungals (ketoconazole, itraconazole), clarithromycin, ritonavir, and cimetidine^[29][14] all require caution, as does grapefruit juice.^[29] Drugs that activate CYP3A4 — rifampicin^[14] and carbamazepine^[9][13] — reduce zopiclone levels and effectiveness.

Alcohol does not meaningfully alter how zopiclone is metabolized, but the combined sedation is dangerous.^[30][14] All CNS depressants — opioids, benzodiazepines, sedating antihistamines, sedating antidepressants — carry the same additive risk.^[29][14][5] Polydrug overdose with CNS depressants is the principal context for fatality.^[5]

Contraindications

• Concurrent CNS depressant use — Additive sedation and respiratory depression with alcohol,^[30][14] opioids, benzodiazepines, and other sedating agents substantially increases overdose risk.
• Breastfeeding — Zopiclone is excreted in breast milk at concentrations approaching those in blood.^[7]
• Severe liver impairment — bioavailability rises to approximately 97% and half-life extends to approximately 8.4 hours; mandatory dose reduction to 3.5 mg.^[12]
• Elderly patients (over 70) — Bioavailability increases up to approximately 168% and half-life extends to approximately 8.1 hours; dose reduction to 3.75 mg required, with elevated falls risk and cognitive vulnerability.^[12][31][32]
• History of substance use disorder — Elevated dependence risk.^[19][33]
• Breathing difficulties — Caution warranted given the mechanism; zopiclone-specific data in chronic lung disease populations are limited.safety citation needed
• Driving and machinery — Avoid for at least 8–10 hours after a 7.5 mg dose; documented residual impairment persists up to 10 hours.^[4][5]
• Pregnancy — Safety has not been established; fetal exposure is presumed given breast milk excretion data.safety citation needed

Zopiclone is a controlled substance in all major jurisdictions, classified alongside benzodiazepines and other sedative-hypnotics.