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Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.

Smoking interactions and cytochrome induction

Psychiatry · UK scope · Sources checked 7 Sept 2026 · Clinical review pending

RapidFull
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Smoking cessation can expose clozapine toxicity

Marked sedation, confusion, myoclonus, seizures or cardiovascular instability after stopping cigarettes may reflect rising clozapine exposure, especially during a concurrent infection.

Action: Arrange urgent clinical assessment and clozapine-team advice, check actual dose and smoking change, obtain a plasma concentration and relevant acute tests, and withhold further doses when toxicity is suspected.

Synopsis

Explain how tobacco smoke changes psychotropic metabolism, anticipate toxicity when smoking stops and loss of effect when it resumes, and coordinate smoking cessation with safe dose monitoring across care transitions.

  • Polycyclic aromatic hydrocarbons in combustible tobacco smoke induce CYP1A2; nicotine itself is not responsible for that enzyme induction.
  • Switching completely from cigarettes to nicotine replacement or an e-cigarette removes the smoke exposure relevant to this interaction.
  • Stopping or substantially reducing smoking can increase concentrations of CYP1A2 substrates such as clozapine and olanzapine.
  • Starting or resuming cigarettes can increase clearance and reduce efficacy, so changes require review in both directions.
  • For clozapine seek urgent specialist advice, obtain levels and repeat approximately one week after cessation-related adjustment.
  • For olanzapine monitor sedation, dizziness and hypotension; SPS advises a 25% dose reduction if adverse effects develop after stopping.
  • Record actual cigarettes smoked and the date of change, rather than copying a binary smoker label.
  • Support cessation actively and manage the interaction through monitoring; do not advise continued smoking as a way to control drug levels.

Key red flags

A person admitted to a smoke-free ward on clozapine needs prompt review of dose and blood levels even if nicotine replacement controls their cravings.

New reduced consciousness, seizure or severe postural collapse after a smoking change requires emergency assessment.

Resuming smoking after discharge can lower antipsychotic exposure and precipitate relapse if the inpatient dose reduction is never reviewed.

Investigation priorities

01
Detailed smoking and nicotine historyFirst step

Quantify the relevant exposure and establish the direction of change.

Management branches

Planned cessationPrepare a medicine monitoring plan

A patient taking a smoking-sensitive medicine intends to stop or substantially reduce cigarettes.

  1. Identify all affected medicines and obtain a baseline clinical assessment and relevant concentration where feasible.
  2. Contact the responsible specialist for clozapine before the change and agree an individual dose and repeat-level plan.
  3. Provide nicotine-withdrawal treatment and explain which adverse effects require urgent contact during the following days.

Key medicines

Clozapine during smoking cessationAdjust the established oral daily dose under urgent specialist supervision using clinical findings and plasma levels; obtain a level at the change and repeat after about one week.There is no universally safe fixed percentage reduction; serious infection, caffeine changes and inhibitors may alter the requirement further, and suspected toxicity needs urgent withholding assessment.
Olanzapine after stopping cigarettesIf increased adverse effects occur, SPS advises reducing the current oral daily dose by 25%; if smoking resumes, titrate towards the previous smoking dose according to response.Seek the prescribing team's input and, where available, compare plasma levels before the smoking change and one week later; do not apply this percentage automatically to clozapine.
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Sources and review status4 sources · checked 7 Sept 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 7 Sept 2026; clinical approval remains outstanding.

  • SPS specific interactions with smokingClozapine and olanzapine adjustment and level timing.
  • SPS considering smoking interactionsCombustion-related enzyme induction and nicotine distinction.
  • Clozaril 25 mg SmPCSmoking cessation, e-cigarettes, infection and concentration monitoring.
  • MHRA smoking cessation medicine interactionsClinically important affected medicines and mechanism.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateRapid draftClinical stateAwaiting reviewJurisdictionUnited Kingdom