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High risk medicines: clozapine

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Clozapine is an antipsychotic medicine used to treat schizophrenia.

But it can cause serious side effects. It is used when:

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


Consultant psychiatrists prescribe clozapine in secondary care.

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

Before starting to prescribe clozapine, the prescriber will complete a physical health check.

People remain under the care of mental health services while they are on clozapine. GPs should make sure people have an annual health check.

People are registered with the clozapine patient monitoring service (CPMS). They will be given a patient registration number and an information pack. The pack should be used to support discussions with the person about safe use of clozapine.

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


People must give informed consent for regular blood testing before prescribing is started. People usually will have a blood test at least every four weeks. This could be more frequent when clozapine is started or if their blood level is not stable.

The blood test is used to check the persons’ white blood cell count. The results are reported by the CPMS in three categories:

  • category one: continue treatment
  • category two: continue treatment with caution and more frequent blood tests
  • category three: 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 CPMS. This may not be your usual pharmacy. The amount of clozapine supplied will be enough for the time period between blood tests.

Side effects

There are many common side effects which can be found on the Patient Information Leaflet (PIL). Staff should check regularly for side effects.

There are some serious side effects that care staff must be aware of. 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
    • as many as a third of people taking clozapine may develop diabetes after 5 years of treatment, the majority of these 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

  • check if a person you are caring for is prescribed clozapine
    • clozapine is prescribed by a hospital prescriber
    • it does not always appear on the GP summary care record.
  • has the person had a physical health check?
    • 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 of 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
    • do you or the person know to seek advice if their smoking status or caffeine intake changes?
  • put support in place for people to have regular blood tests
    • making and attending appointments
    • receiving and acting on results
  • people’s prescriptions and medicines must be available when needed
    • it is important that clozapine is taken as prescribed - 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, both troublesome and potentially serious
    • know who to contact for advice and support
    • know which CPMS the person is registered with
    • care plans should include possible serious side effects that need immediate medical attention.
Last updated:
14 August 2020