Cannabis and Your Medications

As more people turn to medical cannabis for conditions including pain, nausea, and sleep problems, it is important to know if it interacts with any other medicines that you might be taking.

The active ingredients in medical cannabis, primarily THC and CBD, are known to interact with a wide range of prescription medicines. Some people believe, falsely, that as cannabis is a natural product that it does not interact with other medicines.

Most of these interactions are a result of both the medical cannabis and the prescribed medicines being metabolized (broken down) by the liver. More specifically, they are broken down by a group of enzymes in the liver known as the CYP450 system. If these enzymes are busy breaking down the THC and CBD then they have less capacity to break down your prescribed medicines, leading to higher blood levels and potentially causing side effects or toxicity.

It is not just medical cannabis that can do this, as even grapefruit juice can block these receptors and interact with medicines - you may have seen a warning about grapefruit juice on your medicine labels and wondered what it meant. A good tip is that if a medication has a warning on its label to avoid grapefruit, it's a strong indicator that it will also interact with cannabis.

As the CYP450 system in the liver breaks down hundreds of different medicines, there are therefore hundreds of possible interactions with medical cannabis. Some of the most important (as they are common and/or dangerous) are:

• tricyclic antidepressants such as amitriptyline, raising the chances of an increased heart rate, increased blood pressure or more drowsiness
• the epilepsy medicine phenytoin
• the asthma medicine theophylline
• the common blood-thinning medicine warfarin, significantly increasing the risk of internal bleeding and bruising
• sedatives and anxiety medications, leading to excessive sedation and drowsiness, or even confusion & impaired judgment
• blood pressure medications, potentially causing your blood pressure to drop too much

The above describes how medical cannabis can increase the blood levels of other medications, which are probably the most important interactions to be aware of.

However, it is important to note that there are other types of interactions in which the prescribed medicine(s) that you are taking increase or decrease the effect of the medical cannabis.

The effects of medical cannabis can be increased by some medicines - medicines that do this include:

• some antidepressants including fluoxetine (Prozac)
• proton pump inhibitors that are used for gastrointestinal problems such as omeprazole
• antibiotics such as erythromycin
• calcium antagonists used for high blood pressure such as diltiazem and verapamil
• many of the HIV medications
• and, as discussed above, grapefruit juice

The effects of medical cannabis can be decreased by some medicines - medicines that do this include:

• the antibiotic rifampicin
• epilepsy medicines including carbamazepine and phenytoin
• St John’s Wort - a common herbal medicine for depression

It is therefore strongly recommended that, if starting medical cannabis, that you take advice from a doctor or pharmacist about potential medicine interactions.