Clozapine

How it works

What does clozapine do?

Clozapine is an antipsychotic medicine that helps to adjust the levels of dopamine and other chemicals available in your brain.

Clozapine reduces dopamine activity where it is too high, helping with symptoms like hallucinations. It also adjusts dopamine levels in other areas of the brain to improve focus and concentration. Its effects on other brain chemicals make it more effective and help offset some movement- and hormone-related side effects that other antipsychotics can produce.

Clozapine is only used to treat schizophrenia when other medicines have not worked well.

How long does clozapine take to start working?

Clozapine may take several weeks to start working, as the dose starts low and is increased slowly.

How long will I need to take clozapine for?

If clozapine works for you, you may want to keep taking it for a long time.

Your doctor will regularly discuss with you whether you are taking the correct dosage.

Taking clozapine

What if I miss a dose?

If you forget to take a dose, you should take it as soon as possible, unless it is less than four hours until your next dose.

If you forget by the time your next dose is due, just take your next dose.

Do not take a double dose.

What will happen if I forget to take my clozapine?

If you miss more than two days of clozapine (48 hours), you need to start again at a low dose and should speak to your doctor or care team. If you start again at your old dose you may get some intense side effects such as a drop in blood pressure leading to dizziness, unpleasant heart sensations and feeling very tired.

Make sure you take enough medication with you if you are going away.

The information on this page was reviewed by the College of Mental Health Pharmacy in March 2020.

Safety headlines

Clozapine can affect the way your body handles infections. Tell your doctor or care team if you get unexpected fever, sore throat or illness.

It is very important to tell your doctor or care team if you stop or start smoking while taking this medication. Stopping smoking is likely to significantly increase your clozapine levels and lead to side effects. Expect your dose to be reduced.

Clozapine can cause serious side effects including allergic reactions (difficulty breathing, swelling of your face or throat, itching skin lumps), a painful and long-lasting erection (priapism), and other serious symptoms that you can find detailed in the leaflet inside your medication box. Go to a hospital if you get any of these symptoms and take your medication with you.

When to go to the hospital

Clozapine can cause serious side effects including allergic reactions (difficulty breathing, swelling of your face or throat, itching skin lumps), a painful and long-lasting erection (priapism), and other serious symptoms that you can find here. Go to a hospital if you get any of these symptoms, taking your medication along with you.

When to see your doctor

Clozapine can affect the way your body handles infections. Tell your doctor or care team if you get unexpected fever, sore throat or illness.

Tell a doctor straight away, before taking the next clozapine dose, if you:

• have signs of a cold, fever, flu-like symptoms, sore throat or any other infection(You will have to have an urgent blood test to check if your symptoms are related to your medicine)
• suddenly get a high temperature and rigid muscles. This may lead to unconsciousness (neuroleptic malignant syndrome), a serious side effect of this medication
• have crushing chest pain, chest tightness, pressure or squeezing. Chest pain may move from your chest to the left arm, jaw, neck and stomach. You may also get shortness of breath, sweating, weakness, light-headedness, feeling or being sick and palpitations. These might be symptoms of a heart attack
• have a fast and uneven heartbeat, even when you are resting; palpitations; breathing problems; chest pain or unexplained tiredness
• get chest pressure, heaviness, tightness, squeezing, shortness of breath, burning or a choking sensation
• are feeling sick or being sick and do not feel as hungry as normal. This may be a sign that your liver is affected by your medication
• find it difficult to poo for more than a few days (you are very constipated) - do not let constipation go untreated
• get signs of a chest infection such as high temperature, coughing, difficulty breathing and wheezing
• get signs of blood clots in the veins, especially in the legs. The symptoms include swelling, pain and redness in the leg. These can travel through blood vessels to the lungs causing chest pain and difficulty in breathing
• get a lot of sweating, headache, feeling or being sick, and diarrhoea (loose poo)
• are not going to wee very often. This may be a sign that your kidneys are affected 
• are having seizures (fits)
• get an erection that is painful and lasts more than four hours. This is called priapism

It’s also very important to let your doctor know straight away if you start or stop smoking while taking clozapine.

Side effects

Your medicine box contains a leaflet which describes all known side effects for your medication, both rare and common.

If you do get a side effect, please think about reporting it via the 'Yellow Card' scheme.

Some side effects of clozapine should get better after a few days of taking the medication. If they don’t, you should discuss them with your doctor. Expect early drowsiness and dizziness to settle as treatment goes on. This is why doses are built up slowly.

Do not suddenly stop taking your tablets until you have done this, as you may get ‘kick-back’ symptoms as well as losing the benefits of your treatment.

Body

Your weight could be affected by taking clozapine, although it’s very difficult to know how much weight you might gain, if any, as each person is affected differently.

Young people naturally gain a little weight each year as they grow, but anything more than this should be monitored.

Talk to your doctor if you are worried about gaining weight while taking this medication.

Up to one in ten people start to wet the bed when they begin taking clozapine. If this happens to you, restrict the amount that you drink before bedtime and speak to your doctor about other possible solutions, including medications that can stop bed-wetting.

Many people can produce more saliva than usual when they start to take clozapine, which can lead to drooling/dribbling. This usually happens early on in treatment and often gets better after the first few months. If you’re affected, you should talk to your doctor about ways to manage it.

Sex and fertility

Sex

Clozapine can have side effects that affect your sex life. These include:

• an erection that is painful and lasts for a long time (priapism). If you’re affected, you should see your doctor straight away as this can be dangerous.
• bed-wetting.

If any of these symptoms affect you, go back to the doctor and see what can be done.

Clozapine may have a good effect on your sex life as your symptoms settle, and you can concentrate on your relationships.

Fertility

There is nothing to suggest that clozapine has any effect on fertility, although not enough studies have been done to prove this.

If you have taken other antipsychotic medicines before clozapine, your periods may have been affected. People taking clozapine who have periods usually return to a regular period pattern.

Pregnancy, post-natal and breastfeeding

Pregnancy

You should tell your doctor if you are pregnant or trying for a baby before taking clozapine.

If you take clozapine while you are pregnant, it is not thought to have a bad effect on the developing baby.  

Make sure you use a reliable method of contraception while you are taking clozapine if you do not wish to become pregnant. If you’re not sure what method to use, visit your doctor or make an appointment at a sexual health clinic to discuss your options.

Pregnant women are advised to take folic acid – this is safe to do while also taking clozapine.

You may also be checked for increased risk of blood clots during pregnancy and may be prescribed a course of blood-thinning injections.

Post-natal

Clozapine can cause symptoms in newborn babies if you take it during the last few months of pregnancy. These could include shaking, muscle stiffness and/or weakness, sleepiness, agitation, breathing problems and difficulty with feeding. Talk to your doctor or midwife about this and get their help and support.

Breastfeeding

Clozapine passes to the baby in breastmilk and can be harmful.

It can cause problems like excess sleepiness and low white blood cell count.

There is also a very small chance that your baby might have a seizure (fit).

For this reason, it’s best to discuss feeding options with your doctor or midwife.

Driving and transport

You might feel sleepy or dizzy in the first few days after taking clozapine. If this is the case, do not drive a car, ride a bike or operate machinery of any kind until you see how this affects you.

Don’t worry as most people who take clozapine are able to drive or ride a bike as normal after the first few weeks of treatment.

Friends and family

You may want to let your family and friends know you are taking clozapine so they can support you and help you look out for side effects.

For guidance on this, check out our page on getting support with your medication.

Sport

Clozapine is not a banned substance in sport

If you get a side effect like tiredness, dizziness or blurred eyesight, however, this might cause a problem for sports where you need a lot of focus. This is most likely to happen during the first few weeks of taking clozapine.

Try not to worry as most people are able to enjoy the sports they normally would while taking clozapine.

Alcohol and street drugs

Alcohol

You can drink alcohol while taking clozapine, but it could make you very sleepy and you could become unsteady and even take a fall. This will be most noticeable during the first few weeks of treatment.

It’s best to stop drinking alcohol completely during the first few days and weeks, until you see how the medication affects you.

Drinking too much alcohol (getting drunk) while taking clozapine can be very dangerous as it can affect your breathing.

Do not worry - most people are able to enjoy small amounts of alcohol while taking clozapine.

Street drugs

Clozapine does not mix well with street drugs as antipsychotics block the effect of dopamine, meaning that the ‘high’ you experience may not be as potent as before from any drug.

You may be tempted to increase your dose of the drug to make up for this, which could be very dangerous.

Smoking cannabis can make any drowsiness experienced when taking clozapine worse.

Cannabis and other drugs may have their own side effects on your mental health, like anxiety or psychosis. For more information, have a look at our drugs and alcohol page.

Clozapine is also likely to reduce the level of euphoria (happiness) you might normally experience when taking cocaine.

Clozapine and stimulant drugs such as amfetamines can affect the heart when taken together.

Prescription medicines

Tell your doctor or pharmacist if you are taking any other kind of medication as they may need to change your dose of clozapine or prescribe you a different medication.

Other medications can include tablets or liquids that you buy over the counter to treat common illnesses such as colds and flu, or topical medications that are applied to the skin.

Other interactions

Clozapine levels can be changed by high doses of caffeine.

If you drink a lot of coffee, cola drinks or other high-caffeine drinks, it may increase your level of clozapine and this might be dangerous.

If you suddenly stop drinking caffeine, it could make your clozapine level fall.

Talk to your doctor before making any major changes to your diet or what you drink regularly.

Monitoring

You will need regular blood tests all the time you are on clozapine. These blood tests check on the health of your white blood cells, which are important for fighting off infections.

Separate arrangements to check actual blood clozapine levels as a way of helping to get the dose right may be put in place but these tests are not strictly needed.

References and further reading

For more helpful links and information, have a look at our references and further reading page.