Clozapine's dose is cut to a third with fluvoxamine, and should be reduced when a patient stops smoking
The Clozaril label: "Reduce CLOZARIL dose to one-third when coadministered with strong CYP1A2 inhibitors (e.g., fluvoxamine, ciprofloxacin, enoxacin)," and consider reducing the dose when tobacco smoke or carbamazepine is stopped.
Reasonable evidence with real limitations. how we score evidence
Caveat on this rating: The one-third rule is a label instruction, not a measured average; individual changes vary widely, which is why the label pairs it with monitoring.
Clozapine is the antipsychotic reserved for schizophrenia that has not responded to others, and the one whose blood level is most sensitive to what else the patient takes, including cigarettes.
Significant findings
CLOZARIL prescribing information (HLS Therapeutics, June 2025), Drug Interactions summary: "Concomitant use of Strong CYP1A2 Inhibitors: Reduce CLOZARIL dose to one-third when coadministered with strong CYP1A2 inhibitors (e.g., fluvoxamine, ciprofloxacin, enoxacin)." "Concomitant use of Strong CYP3A4 Inducers is not recommended." "Discontinuation of CYP1A2 or CYP3A4 Inducers: Consider reducing CLOZARIL dose when CYP1A2 inducers (e.g., tobacco smoke) or CYP3A4 inducers (e.g., carbamazepine) are discontinued." "Anticholinergic drugs: Concomitant use may increase the risk for anticholinergic toxicity."
Section 7.1: "Clozapine is a substrate for many cytochrome P450 isozymes, in particular CYP1A2, CYP3A4, and CYP2D6. Use caution when administering CLOZARIL concomitantly with drugs that are inducers or inhibitors of these enzymes." And: "The CLOZARIL dose should be increased to the original dose when coadministration of strong CYP1A2 inhibitors is discontinued".
Section 7.2 runs the other way: clozapine "can increase levels of these CYP2D6 substrates," including "specific antidepressants, phenothiazines, carbamazepine, and Type 1C antiarrhythmics".
The boxed warning on the same label, about neutropenia, low blood pressure and seizures, is why the level matters: seizures and sedation rise with the level, and the level can triple when fluvoxamine is added or when a smoker stops.
Worth asking
The interaction most likely to catch someone is the one with no pill: stopping smoking, including on admission to a smoke-free ward, removes the induction and the clozapine level climbs. If you take clozapine and are quitting, or are prescribed the antibiotic ciprofloxacin, ask whether a clozapine level and a dose review are planned. The Haslemo 2006 row in this wave measured how much smoking changes the level.
Source
CLOZARIL (clozapine) tablets: FDA-approved prescribing information, Dosage 2.7, Drug Interactions 7.1 and 7.2 — HLS Therapeutics (USA), Inc.; US Food and Drug Administration (2025)
Regulator or national health body
How this was scored
- Study design
- Regulatory safety determination
- Funding
- Industry funded
- Published in
- Regulator or national health body
- Sample size
- not recorded
- 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 "moderate evidence". Reasonable evidence with real limitations. It scores 57 out of 100 on our published rubric. One caveat travels with that badge: The one-third rule is a label instruction, not a measured average; individual changes vary widely, which is why the label pairs it with monitoring. 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?
CLOZARIL (clozapine) tablets: FDA-approved prescribing information, Dosage 2.7, Drug Interactions 7.1 and 7.2 — HLS Therapeutics (USA), Inc.; US Food and Drug Administration (2025). Published in: Regulator or national health body. 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: Regulatory safety determination. Funding: Industry funded. 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.