Medical cannabis and medications: understanding possible interactions

Using cannabis for medical purposes involves more than choosing an oil, capsule or dried flower. Prescription medicines, over-the-counter products, natural health products, alcohol and other substances must also be considered. An interaction does not occur in every case, but it may change the intensity or duration of an effect and increase certain risks.

What is a drug interaction?

An interaction occurs when one substance influences how another acts in the body. It may be pharmacodynamic, when similar effects add together, such as drowsiness. It may also be pharmacokinetic, when a substance changes the absorption, metabolism or elimination of another. THC and CBD do not behave in the same way, and risk depends on factors such as concentration, dose, route of administration, frequency of use and the individual's circumstances.

Which medications and products should be disclosed?

Health Canada recommends telling your health care professional about every medication and health product you use, even when it seems unrelated to cannabis. Its list of possible interactions includes anticoagulants, antidepressants and anti-anxiety medicines, anti-seizure medicines, antipsychotics, immunosuppressants, some heart medicines, some antibiotics or antifungals, and products that can cause drowsiness. This list identifies possibilities to assess; it does not mean every combination will cause an interaction in every person.

• Prescription medicines: — bring an up-to-date list with the name, dose and time each medicine is taken.
• Over-the-counter products: — include medicines used for allergies, colds, pain or sleep.
• Natural health products: — mention supplements and herbal products, including St. John's wort.
• Other substances: — disclose alcohol, nicotine and anything else that could affect alertness or change the experience.

Why drowsiness deserves particular attention

Cannabis may cause drowsiness, dizziness, reduced attention or impaired coordination. These effects may add to those of sleeping pills, tranquilizers, some antihistamines, muscle relaxants or pain medicines. Health Canada also advises avoiding cannabis with alcohol. Anyone experiencing impairment should not drive or operate machinery.

CBD does not mean interaction-free

CBD does not generally cause the intoxication associated with THC, but it is not inactive. Health Canada notes that CBD and THC can influence liver enzymes involved in the metabolism of many medicines. Clinical evidence remains incomplete, and the practical importance varies by product and person. A product marketed as CBD-rich may also contain THC, so the label and the licensed producer's certificate of analysis should be reviewed.

Which changes should prompt a review?

A review is especially relevant when a new medication is added, a prescription dose changes, a cannabis product or its THC-to-CBD ratio is changed, or new effects appear. Unusual drowsiness, pronounced dizziness, confusion, palpitations or any other concerning effect are reasons to contact a health care professional promptly. Call 911 for a serious effect or emergency.

How to prepare for your next follow-up

Before the appointment, record the exact cannabis product name, its THC and CBD concentrations, the amount used, the time it is taken and the effects observed. Add your full medication list and any recent changes. Do not enter this medical information in Coverleaf's public callback form: it asks only for contact details, the best time to reach you and the general topic of your request. The team will then explain the appropriate channel for clinical follow-up.

What the practitioner may reassess

The practitioner may review the product, THC-to-CBD ratio, route of administration, timing, adverse effects and other medications. A pharmacist can also help assess the medication profile. Do not change a prescribed medicine or your medical cannabis plan on your own based on a list found online: the decision depends on your record and an individualized assessment.

Primary sources

• Health Canada, Cannabis for medical purposes - General information: — https://www.canada.ca/en/health-canada/topics/accessing-cannabis-for-medical-purposes/cannabis-medical-purposes.html
• Health Canada, Consumer information - Cannabis: — https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/laws-regulations/consumer-information.html
• Health Canada, Cannabis and cannabinoids - health care professionals: — https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/information-medical-practitioners/information-health-care-professionals-cannabis-cannabinoids.html