A clozapine patient became confused after quitting tobacco and cannabis: the smoke had been holding his level down
Two cases: a clozapine patient "developed confusion after tobacco and cannabis smoking cessation, which was related to increased clozapine plasma levels"; an olanzapine patient developed marked movement side effects after cutting down.
Weak or heavily disputed. Treat as a question, not an answer. how we score evidence
Caveat on this rating: Two case reports; the abstract attributes the effect to smoke constituents without separating cannabis from tobacco, so the cannabis contribution on its own is inferred, not measured.
Cannabis is usually smoked, and smoke of any kind changes how the body handles two of the most used antipsychotics. This report is the interaction running backwards, when the smoking stops.
Significant findings
Zullino and colleagues (International Clinical Psychopharmacology, 2002): "Plasma levels of clozapine and olanzapine are lower in smokers than in nonsmokers, which is mainly due to induction of cytochrome P4501A2 (CYP1A2) by some smoke constituents. Smoking cessation in patients treated with antipsychotic drugs that are CYP1A2 substrates may result in increased plasma levels of the drug and, consequently, in adverse drug effects."
"Two cases of patients who smoked tobacco and cannabis are reported. The first patient, who was receiving clozapine treatment, developed confusion after tobacco and cannabis smoking cessation, which was related to increased clozapine plasma levels. The second patient, who was receiving olanzapine treatment, showed important extrapyramidal motor symptoms after reducing his tobacco consumption."
The clinical rule they draw: "smoking patients treated with CYP1A2 substrate antipsychotics should regularly be monitored with regard to their smoking consumption in order to adjust doses in cases of a reduction or increase in smoking."
Worth asking
The clozapine label and the Haslemo 2006 study in the interactions wave give the size of the tobacco effect: roughly double the level per milligram in non-smokers. Cannabis smoke contains the same enzyme-inducing hydrocarbons, so a person who smokes cannabis heavily and then stops, or is admitted somewhere they cannot, may see their clozapine or olanzapine level climb over a week or two. The question for a prescriber is whether a level is checked whenever smoking of any kind changes, and the question for the patient is to say so when it does.
Source
Tobacco and cannabis smoking cessation can lead to intoxication with clozapine or olanzapine — Zullino DF, Delessert D, Eap CB, Preisig M, Baumann P (2002)
Reputable peer-reviewed journal
Read the source: https://doi.org/10.1097/00004850-200205000-00008
DOI: 10.1097/00004850-200205000-00008
How this was scored
- Study design
- Case series or case report
- Funding
- Funding not disclosed
- Published in
- Reputable peer-reviewed journal
- Sample size
- 2
- Preregistered
- not recorded
- Conflicts disclosed
- not recorded
- Independent of proponent
- not recorded
- Retracted
- No
Read the full scoring rubric, including what it can't tell you.
Published September 21, 2026.
Questions
How strong is the evidence behind this?
veisund rates this source "contested". Weak or heavily disputed. Treat as a question, not an answer. It scores 32 out of 100 on our published rubric. One caveat travels with that badge: Two case reports; the abstract attributes the effect to smoke constituents without separating cannabis from tobacco, so the cannabis contribution on its own is inferred, not measured. The score is calculated from recorded facts about the source — study design, funding, publication venue, sample size, preregistration — not typed in by an editor.
What is the source for this?
Tobacco and cannabis smoking cessation can lead to intoxication with clozapine or olanzapine — Zullino DF, Delessert D, Eap CB, Preisig M, Baumann P (2002). Published in: Reputable peer-reviewed journal. DOI: 10.1097/00004850-200205000-00008. The full source is linked on this page so you can read it yourself.
Who paid for this research, and does that matter?
Study design: Case series or case report. Funding: Funding not disclosed. Independence from the proponent is not recorded. Industry sponsorship is one of the most reliably measured biases in medicine, which is why funding carries real weight in the score rather than sitting in a footnote.
Is this medical advice?
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.