HeadMeds gives young people in the United Kingdom general information about medication. HeadMeds does not give you medical advice. Please talk to your Doctor or anyone else who is supporting you about your own situation because everyone is different. Please read more important details about our site.

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Return to Clozapine overview
  1. Use and Action
  2. Warnings and side effects
  3. Sex, drink, weight and everything else

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 which may lead to unconsciousness (neuroleptic malignant syndrome) as you may be experiencing a serious side effect which needs treatment straight away.
  • 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. You need to get emergency medical treatment straight away.
  • have a fast and uneven heartbeat, even when you are resting, palpitations, breathing problems, chest pain or unexplained tiredness. Your doctor will need to check your heart.
  • get chest pressure, heaviness, tightness, squeezing, shortness of breath, burning or choking sensation. Your doctor will need to check your heart.
  • are feeling sick or being sick, and do not feel as hungry as normal. Your doctor will need to check your liver.
  • find it difficult to poo for more than a few days (you are very constipated).
  • get signs of a chest infection such as high temperature, coughing, difficulty breathing, 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 loose poo (diarrhoea).
  • are not going to wee very often. Your doctor needs to check your kidneys
  • are having fits (seizures).
  • are a man and get an erection that is painful and lasts for than 4 hours. This is called priapism. You need to go to a hospital straight away to get help.

Clozapine may take several weeks to start working

  • Clozapine may take several weeks to start working as the dose starts low and increases to the regular dose slowly. Symptoms may continue to get better for many months.

You may keep taking clozapine for a long time

  • If clozapine works for you, you may want to keep taking it for a long time.
  • You might want to discuss changing the dose with your doctor if your symptoms are stable for some time.

If you forget to take a dose take it as soon as possible unless it is getting close to your next dose

  • If you forget a dose 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.
  • If you miss more than 2 days of clozapine (48 hours) you need to start again at a low dose and you 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 drop in blood pressure leading to dizziness, unpleasant heart sensations and you are likely to feel very tired.
  • Make sure you take enough medication with you if you are going away.

You will need regular blood tests all the time you are on clozapine

  • Your care team should have explained all about this to you.
  • Clozapine has many side effects for details look at the information leaflet in the pack or discuss with your doctor, care team or pharmacist. 
  • These blood tests check on the health of your white blood cells- 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 absolute requirements.