High risk medicines: clozapine

Page last updated: 20 March 2023
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Clozapine is an antipsychotic medicine used to treat psychosis, including schizophrenia and psychosis in Parkinson’s disease.

It is used when either:

  • at least 2 other treatments have not worked
  • a person is unable to take other antipsychotics because of their side-effects.


A specialist psychiatric consultant will start the prescribing of clozapine. Before starting to prescribe clozapine for a person, the prescriber will carry out a physical health check.

People might start to take clozapine either as a hospital inpatient or with support from a community mental health team.

They remain under the care of mental health services while they are taking clozapine, but they should have an annual health check with their GP.

When a person has been prescribed clozapine, they will be registered with the relevant clozapine patient monitoring service. They will receive a patient registration number and an information pack. The pack should be used to support discussions with the person about using clozapine safely.

People should be maintained on one brand of clozapine. They will be registered with the monitoring service of that brand.


Before starting to prescribe clozapine, people should give informed consent for regular blood testing if they are well enough to do so. Blood tests are usually carried out at least every 4 weeks. Tests could be more frequent when a person first starts taking clozapine or if their blood level is not stable.

The blood test is used to check the person’s white blood cell count. The results are reported by the clozapine patient monitoring service in 3 categories:

  • Green - category 1: continue treatment
  • Amber - category 2: continue treatment with caution and more frequent blood tests
  • Red - category 3: stop treatment with clozapine immediately

People will receive a supply of clozapine if the blood test shows that treatment can continue. Clozapine will be supplied from a clinic or pharmacy registered with the relevant clozapine patient monitoring service. This may not be their usual pharmacy.


There are many common side-effects of clozapine, which are explained in the patient information leaflet (PIL). Staff should check regularly for side-effects.

Staff must be aware of some serious side-effects. These include:

  • Blood disorders: signs of problems include flu-like symptoms such as sore throat and temperature.
  • Seizures.
  • Heart disease: signs of a problem include chest pain, palpitations, or a rapid pulse.
  • Diabetes: a third of people taking clozapine may develop diabetes after 5 years of treatment, the majority of these cases happen within the first 6 months.
  • Bowel obstruction: clozapine can cause slowing of bowel movement resulting in constipation, blockage and a ‘paralytic ileus’ which may be fatal. Staff should make sure people are not constipated. A laxative could be prescribed, if needed.
  • Skin reactions: people taking antipsychotics may be more sensitive to sunlight. You should consider how to protect people’s skin from direct sunlight.

Considerations for providers and staff

  • Follow good practice for medicines reconciliation and check if a person you are caring for is prescribed clozapine.
    • clozapine is prescribed by a specialist mental health team
    • it does not always appear on the GP summary care record or repeat prescription
    • make sure you accurately record clozapine on the medicines administration record
  • Check whether the person has had a physical health check within the past 12 months.
    • If the person consents, are you aware of any issues raised from health checks?
    • Is there any further action?
  • Where appropriate, record the person's smoking status and caffeine intake:
    • smoking and caffeine can both cause changes to levels of clozapine in the blood
    • stopping smoking or increasing caffeine intake both cause an increase level of clozapine in the blood
    • the dose of clozapine may need to be changed if there is a change in smoking or caffeine drinking habit. If this happens you need to get specialist advice.
  • Enable support for people to have regular blood tests, including:
    • making and attending appointments
    • receiving and acting on results
  • People’s medicines must be available when needed.
    • It is important that clozapine is taken as prescribed as it can be dangerous to miss doses and then restart at the full dose
    • Seek advice if more than one dose is missed - treatment may need to be restarted at a lower dose and closer monitoring may be needed.
  • Know how to identify possible side-effects.
    • Care plans should include possible serious side-effects that need immediate medical attention.
  • Know who to contact for advice and support and which clozapine patient monitoring service the person is registered with.