Can I take Cannabis and Clozapine together?

Why this happens

Cannabis smoke contains chemicals (hydrocarbons) that speed up the liver enzymes responsible for breaking down clozapine. This can lower the amount of medicine in your system. Conversely, if you stop smoking, the levels can rise rapidly. Additionally, both substances affect the central nervous system and heart rate, leading to 'additive' effects where the side effects of both are intensified.

What you should do

You must speak with your doctor or psychiatrist before using cannabis or if you plan to change your current usage habits. Your doctor will likely need to perform more frequent blood tests to monitor your clozapine levels and may need to adjust your dose. Do not stop or start using cannabis abruptly without medical supervision.

Important precautions

Infection or flu-like symptoms

You must have regular blood tests because clozapine can cause a dangerous drop in white blood cells (agranulocytosis). Contact your doctor immediately if you develop a sore throat, fever, or any signs of infection.

Smoking status changes

If you stop or start smoking tobacco, you must tell your doctor immediately. Tobacco smoke changes how your body breaks down clozapine; stopping smoking can cause clozapine levels to rise to toxic, dangerous levels.

Myocarditis (heart muscle inflammation)

Clozapine carries a risk of serious heart inflammation, especially in the first two months. Contact your doctor immediately if you experience a persistent fast heartbeat at rest, chest pain, or shortness of breath.

Constipation

Clozapine can severely slow down your bowels (paralytic ileus), which can be fatal. You must tell your doctor if you become constipated or have not had a bowel movement for more than two days.

Psychosis or Schizophrenia

You must not use cannabis if you have a personal or strong family history of schizophrenia or other psychotic disorders, as it can trigger or worsen these conditions.

Driving and operating machinery

It is an offence to drive if your ability is impaired by cannabis. Even if you do not feel 'high', the drug stays in your system and can result in a drug-driving conviction.

Food and drink warnings

Caffeine (Coffee, Tea, Cola, Energy drinks)

Caffeine can significantly increase the levels of clozapine in your blood, which may increase the risk of side effects. If you suddenly stop drinking caffeine, your clozapine levels may drop, making the medicine less effective. Try to keep your caffeine intake consistent and talk to your doctor before making any major changes.

Alcohol

Alcohol can increase the sedative effects of both clozapine and cannabis, making you feel very sleepy, dizzy, or confused. It can also increase the risk of falls and impair your coordination. It is best to avoid alcohol while taking these medicines.

Grapefruit juice

Grapefruit juice can interfere with how your body breaks down certain compounds in cannabis (like CBD and THC). This may increase the levels of the medicine in your blood and lead to more side effects.

High-fat meals

Eating a meal high in fat (such as a cooked breakfast or fast food) just before or after taking oral cannabis can significantly increase the amount of the medicine absorbed by your body. This may lead to stronger or more unpredictable effects.

Using other medicines

Taking multiple medicines? Our Medicines Interaction Checker helps you check whether your prescriptions, over-the-counter medicines, and supplements are safe to take together. Simply search for your medicines to see potential interactions and what to do about them.

Disclaimer

This information is for general educational purposes only and should not be relied upon as a substitute for professional medical advice. Always consult your GP, pharmacist, or another qualified healthcare professional before making decisions about your medications. Individual circumstances may vary, and only a healthcare professional who knows your medical history can provide personalised guidance.