Mental health
Clozapine
An antipsychotic for treatment-resistant schizophrenia — The most effective drug for treatment-resistant schizophrenia — used within a strict monitoring scheme.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Clozapine is reserved for schizophrenia that has not responded adequately to other antipsychotics, and sometimes for psychosis in Parkinson's disease. It is highly effective but requires structured safety monitoring.
How it works
Like other antipsychotics it acts on dopamine and serotonin and other receptors, but with a distinctive profile that helps where other drugs fail. The same broad receptor activity, together with its specific effects on blood cells, the heart and the bowel, is why it needs such careful supervision.
In practice
In practice clozapine stands apart: it is the one antipsychotic with proven superiority in schizophrenia that has not responded to other drugs, but its use is governed by a mandatory blood-monitoring scheme because it can cause agranulocytosis — a dangerous fall in infection-fighting white cells. Patients are registered with a monitoring service and cannot receive the drug without an in-range blood count, frequently at first and less often later. Several other serious effects shape day-to-day care: severe constipation that can progress to life-threatening bowel obstruction (it is actively asked about and treated), myocarditis especially in the early weeks, seizures at higher doses, and significant weight gain and glucose problems. A crucial practical point is smoking: tobacco smoke lowers clozapine levels, so starting or stopping smoking — including a hospital admission where someone suddenly stops — can swing levels dangerously and needs dose review. Doses are never restarted at full strength after even a couple of missed days; re-titration is required.
Examples
Practical use
How to take it & use it well
- Take clozapine exactly as prescribed and attend all your blood-monitoring appointments, as treatment can only continue if your blood counts stay safe.
- Never miss the required blood tests, because clozapine is only supplied when monitoring confirms your white cells are at a safe level.
- Tell your team straight away if you develop a sore throat, fever or any sign of infection, as this needs an urgent blood check.
- Report constipation early and keep well hydrated and active, since serious bowel problems can develop and must be treated promptly.
- Tell your prescriber if you start, stop or change how much you smoke, as this strongly affects your clozapine level and may need a dose change.
- If you miss your doses for more than a couple of days, do not simply restart your usual amount; contact your team, as the dose usually has to be built up again slowly.
Common uses
- Treatment-resistant schizophrenia
- Where other antipsychotics have not worked or caused severe movement effects
- Psychosis in Parkinson's disease (specialist)
Monitoring
- Regular white-cell/neutrophil counts via the monitoring service (frequent at first)
- Bowel function, cardiac symptoms (early), weight, glucose and lipids
- Clozapine level where relevant (e.g. smoking change, toxicity, poor response)
Weighing it up
Advantages & disadvantages
Advantages
- Can be highly effective for schizophrenia that has not responded to other antipsychotic medicines.
- May reduce the risk of suicidal behaviour in some people with severe mental illness.
- Can improve quality of life and function when other treatments have failed.
- Closely monitored, which means problems are more likely to be picked up early.
Disadvantages
- Requires regular, mandatory blood tests throughout treatment, which some people find demanding.
- Commonly causes constipation that can rarely become a serious bowel obstruction if not managed.
- Can affect the heart, including a rare inflammation of the heart muscle (myocarditis), especially in the early weeks.
- May cause drowsiness, drooling, weight gain, raised blood sugar and a drop in blood pressure, among other effects.
Key safety principles
What to watch for
- Agranulocytosis — mandatory registered blood monitoring; never given without an in-range count.
- Severe constipation (risk of bowel obstruction), myocarditis (especially early), and seizures at higher doses.
- Smoking changes clozapine levels — starting/stopping smoking needs dose review; re-titrate after missed doses; marked weight and glucose effects.
Key interactions
What to avoid or check alongside
- Stopping or cutting down smoking can raise clozapine levels and increase side effects, while starting smoking lowers them.
- Some antibiotics and antidepressants, such as ciprofloxacin and fluvoxamine, can raise clozapine levels and increase toxicity.
- Other medicines that can lower the white-cell count, such as carbamazepine, are avoided because they add to clozapine's blood-count risk.
- Combining clozapine with strong sedatives or some painkillers can increase drowsiness and slowed breathing.
- Medicines that slow the bowel, including some painkillers and anticholinergic drugs, can worsen clozapine-related constipation.
Patient & carer advice
- You must attend your blood tests — the medicine depends on them
- Tell us urgently about a sore throat or fever, constipation, chest pain or breathlessness
- Tell us if you start or stop smoking, and never just restart after missed doses
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Clozapine: frequently asked questions
Why does clozapine need regular blood tests?
Clozapine can rarely cause a serious fall in infection-fighting white blood cells. Mandatory blood monitoring catches this early, and the medicine is only supplied while your counts remain safe.
What should I do if I miss several days of clozapine?
Do not just restart your usual amount. After a gap of more than a couple of days, the dose usually has to be built back up slowly, so contact your team before taking it again.
How does smoking affect clozapine?
Tobacco smoke speeds up how the body clears clozapine, so smokers often need more. Cutting down or stopping can raise levels and side effects, so always tell your team about changes in smoking.
Why is constipation taken so seriously with clozapine?
Clozapine slows the bowel and can rarely cause a dangerous blockage. Report constipation early, stay hydrated and active, and your team can suggest treatment to keep your bowels moving.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
Related guides
Need a custom medicines or prescribing resource?
We build evidence-led clinical references, calculators and decision aids for teams.