Clozapine is used to treat and prevent the return of symptoms in Schizophrenia and other mental health related conditions. Learn how to use the medication, its common side effects, special precautions to watch out for, and more.

What is this medication for?

Clozapine belongs to a class of medications known as atypical (second-generation) antipsychotics. It is used to treat and prevent the return of symptoms in Schizophrenia and other mental health related conditions such as:

• Hearing, seeing or sensing things that are not real
• Having mistaken beliefs
• Uncontrolled anger
• Being overly suspicious

Clozapine may be used for other conditions. Check with your healthcare professional if you are unsure why you are given this medication.

How should I take or use this medication?

• Take your medication according to the instructions stated on the medication label.
• Clozapine may be taken with or without food. 
• Do not stop taking or adjust the dose of this medication without checking with your doctor.
• Clozapine needs to be taken regularly for a few weeks before you can feel its full benefits. You should not stop taking this medication even if you do not feel better at the beginning. Continue to take this medication even after you feel better to prevent your symptoms from returning.
• Do check with your doctor on how long you will have to be on this medication.

What should I do if I forget to take or use this medication?

• If you forget to take a dose, take it as soon as you remember. However, if more than half of the time to the next dose has passed, skip the missed dose and take the next dose at your usual timing.
• For example, for a medication to be taken two times daily at 8am and 8pm, and you missed your first dose at 8am, skip the first dose if it is after 2pm, and take your second dose at the usual timing at 8pm. Do not take two doses to make up for the missed dose.

If, for any reason, you stop taking clozapine for more than 2 days, do not start back on the same dose. Ask your doctor what dose you should take. If you have problems remembering your doses, speak to your pharmacist or doctor.

What precautions should I take when taking or using this medication?

Inform your healthcare professional if:

• You are allergic to this medication or any of the other ingredients of this medication
• You are pregnant, planning to become pregnant, or breastfeeding. If you are pregnant and/or breastfeeding, your doctor would have discussed the potential benefits and side-effects with you and should you have further questions or concerns, do consult your doctor.
• You are taking any other medications (including those that can be bought without a prescription), supplements, traditional Chinese medicine and herbal products. Some of them may affect how this medication works or cause serious drug interactions.
• You have diabetes. Clozapine may sometimes worsen blood sugar control in diabetes. Maintain regular check-ups with your doctor.
• You are smoking or have any changes to your smoking habits. Smoking may decrease the amount of Clozapine that stays in your body.
• You consume caffeinated food/drinks or have any changes to your caffeine intake. Caffeine may increase the amount of clozapine that stays in your body.
• You have or have ever had:
  • Other drug allergies
  • Medical conditions such as:
    • Blood disorders, irregular heartbeat or other heart problems, urinary, stomach, intestinal or prostate problems, kidney, liver or bone marrow disease, stroke
    • Seizures (fits), glaucoma (high pressure in the eye), high or low blood pressure, history of blood clots, high cholesterol/ fats in the blood, Parkinson’s disease, dementia, sleep apnea (sleep problem where the breathing repeatedly stops and starts during sleeping)

IMPORTANT:

In rare cases, clozapine can cause agranulocytosis, a condition whereby there is a significant reduction in your white blood cells (WBC). This can lead to serious or sometimes life-threatening infections. Symptoms may include fever, sore throat and/or mouth ulcers that do not go away.

To prevent this from happening, your doctor will monitor the levels of your WBC with regular blood tests while you are on clozapine treatment – weekly for the first 18 weeks after starting treatment, then at least monthly thereafter.

If your WBC levels drop too low, your doctor may stop clozapine treatment. WBC levels usually return to normal after stopping clozapine. Do not restart clozapine without your doctor’s advice.

It is therefore important to keep all appointments with your doctor – taking clozapine without regular blood tests can be very dangerous.

Be sure that your family or caregiver are also aware of the above symptoms so they can call your doctor if you are unable to seek treatment on your own. 

What are some common side effects of this medication?

The mentioned side effects generally affect up to 10% of patients.

• Drowsiness or feeling tired
  • Avoid driving or taking part in activities that require concentration
  • Discuss with your doctor if you can take your medication at a different time of the day
• Nausea or, vomiting
  • Take the medication with or after food
  • Take small but more frequent meals, and avoid fatty or spicy food
• Constipation 
  • Drink more water (if you do not have any fluid restriction), eat more high-fibre foods, and exercise regularly
  • Speak to your doctor or pharmacist if you need medication to help with your constipation
• Dizziness
  • Get up slowly from a sitting or lying down position
• Increase in appetite, weight gain
  • Exercise regularly and eat a healthy and balanced diet
• Excess saliva production or drooling
  • Raising your pillows while sleeping may help with drooling

Most of the side effects listed here will improve with time. Speak with your doctor if any of them continues, worsens or affects your daily life.

What are some rare but serious side-effects that I need to seek medical advice immediately?

The symptoms of a drug allergy include one or more of the following: 

• Swollen face/eyes/lips/tongue
• Difficulty in breathing
• Itchy skin rashes over your whole body

Rarely this medication may cause:

• Fever, chills, sore throat and/or mouth ulcers that do not go away (symptoms of blood dyscrasias)
• Seizures (fits)
• Sudden weakness or numbness in the face, arms or legs, and speech or vision problems (signs and symptoms of stroke)
• Confusion, high fever, severe muscle stiffness, irregular blood pressure (signs and symptoms of neuroleptic malignant syndrome)
• Fast or uneven heartbeat, chest pain, shortness of breath (signs and symptoms of myocarditis or cardiomyopathy)
• Fast or irregular heartbeat, fainting episodes (signs and symptoms of an abnormal heart beating rhythm (prolonged QTc interval))
• Severe chest pain, difficulty breathing, or severe pain or swelling in either of the legs (symptoms of venous thromboembolism)
• Difficulty controlling or passing urine (urinary incontinence or retention)
• Excessive hunger, unusual increase in thirst or passing large amount of urine (symptoms of high blood sugar)
• Muscle movement side effects (extrapyramidal side effects) – do speak to your doctor for further management. Examples include:
  • Muscle stiffness and spasms (such as in the face, neck and back), twitching or difficulty in speaking or swallowing (symptoms of dystonia)
  • Restlessness or need to keep moving constantly (akathisia)
  • Uncontrollable shaking of limbs (tremors)
• Severe constipation, vomiting, stomach pain, cramps or bloating, unable to pass gas (signs and symptoms of intestinal obstruction)

If you experience any of these symptoms, you should stop your medication and see your healthcare professional immediately.

What food or medication should I avoid when I take or use this medication?

Avoid drinking alcohol while taking Clozapine as it can cause excessive sleepiness. 

How should I store this medication?

Store in a cool and dry place, away from direct sunlight. Keep this medication away from children. 

How do I throw away this medication safely?

Pack this medication into a trash bag and seal it tightly before throwing into the rubbish chute or bin.

Disclaimer

Please take note that the above is not a complete list of all possible side effects. If you have any concerns about your medication or if you have other side effects that you think are caused by this medication, please consult your doctor or pharmacist.

If you take more than the recommended dose, please seek medical advice immediately. The information provided on this page does not replace information from your healthcare professional. Please consult your healthcare professional for more information.

This article is jointly developed by members of the National Medication Information workgroup. The workgroup consists of cluster partners (National Healthcare Group, National University Health System and SingHealth), community pharmacies (Guardian, Unity and Watsons) and Pharmaceutical Society of Singapore. The content does not reflect drug availability and supply information in pharmacies and healthcare institutions. You are advised to check with the respective institutions for such information.

Last updated on Aug 2023