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Cannabis drug interactions: what to know about other medicines
THC and CBD are broken down in the liver by the same enzymes as many other medicines and can change their levels. Added to this are increased tiredness and effects on circulation. Which combinations need particular caution.
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Medical cannabis does not act in isolation. If you take other medicines, you should know about possible interactions and have every combination checked by a doctor. Two mechanisms matter: changes in how medicines are broken down in the liver, and effects that reinforce each other.
How do interactions arise?
THC and CBD are broken down in the liver by enzymes of the cytochrome P450 system, mainly CYP3A4, CYP2C9 and CYP2C19. Many other medicines are metabolised by the same enzymes. CBD can inhibit some of these enzymes, which can raise the levels of other medicines.
There are also pharmacodynamic interactions: two medicines that make you tired or lower blood pressure reinforce each other.
What you leave out cannot be taken into account. Honesty is part of the treatment.
Which combinations need particular caution?
Blood thinners such as warfarin or phenprocoumon: clotting values can change, so close monitoring is needed.
Certain antiepileptics such as clobazam or valproic acid: levels and liver values can change.
Sedatives and sleeping pills, opioids, some antihistamines and alcohol: increased tiredness, dizziness and a higher risk of falls.
Antidepressants and antipsychotics: possible changes in levels and effects, plus mental effects. Heart and blood pressure medicines as well as immunosuppressants after a transplant are also among the combinations a doctor checks closely.
What should you do?
In your request, list all medicines with their dosage, including over-the-counter products, herbal remedies such as St John's wort and dietary supplements. Do not add or stop any medicine on your own. Also tell your pharmacy (Apotheke) that you take medical cannabis.
When does particular caution apply?
With heart and circulation diseases, liver diseases, in older age and when many medicines are taken at the same time. The adjustment is then particularly slow and checks are more frequent; more on this in Medical cannabis dosage.
Interactions and driving
Combinations that make you tired impair your fitness to drive. The exemption for prescribed medicines in German road traffic law (Arzneimittelprivileg) does not protect you if you cannot drive safely. More in the guide Driving with a cannabis prescription.
Frequently asked questions
Can I combine cannabis with antidepressants?
Only after checking with your doctor. Levels and effects can change.
What about painkillers?
Opioids in particular can cause more tiredness together with cannabis. The combination needs a careful adjustment.
Do I also have to mention dietary supplements?
Yes. Herbal remedies such as St John's wort also affect how medicines are broken down.
Does CBD have fewer interactions than THC?
No, on the contrary: CBD inhibits some liver enzymes more strongly and is therefore particularly relevant for interactions. More in What does CBD do?
Sources
420doc editorial team · Published on 26 October 2026 · Updated on 26 October 2026
This article is for general information only and does not replace individual medical advice. How we review our content
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You will find more background in the magazine and the glossary. Whether a treatment suits you can only be clarified in a personal conversation with a doctor.
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