Medical Cannabis and Prescription Medications: What You Need to Know About Drug Interactions
If you are considering medical cannabis, or already using it, one of the most important things your healthcare provider needs to know is what else you are taking.
That includes prescription and over-the-counter medications, vitamins, supplements, natural health products, alcohol, and other substances.
Cannabis can interact with certain medications in ways that may increase side effects or affect how a medication behaves in the body. This doesn’t mean everyone taking prescription medication needs to avoid cannabis. It means your medications should be considered as part of your overall medical cannabis assessment.
At Greenleaf Medical Clinic, reviewing your medical history, symptoms, current treatments, and medications helps our physicians evaluate whether medical cannabis may have an appropriate role in your care.
How Can Cannabis Interact With Medication?
Cannabis contains numerous compounds, including THC (delta-9-tetrahydrocannabinol) and CBD (cannabidiol). Both can have therapeutic effects and both have the potential to interact with medications.
The effects of THC are dose-dependent and can vary considerably between individuals. In medical cannabis care, THC may be introduced cautiously and used at low or sub-perceptual levels, depending on the individual treatment plan and therapeutic goals. At higher doses, THC can produce noticeable psychoactive effects and impairment.
CBD is not intoxicating, but that does not mean it is inactive or unable to interact with medications. CBD can influence several CYP450 enzymes involved in medication metabolism. The likelihood and clinical significance of an interaction can vary depending on the medication, the amount of CBD being used, and individual patient factors.
That’s why it’s important to tell your healthcare provider about all cannabis products you use—including CBD products and THC-containing cannabis.
Cannabis and medications can generally interact in two important ways.
1. Cannabis May Affect How Your Body Processes Certain Medications
Your liver plays an important role in breaking down medications and other substances. One of the systems involved in this process is the cytochrome P450 (CYP450) enzyme system.
What Are CYP Enzymes?
CYP enzymes are a family of enzymes found primarily in the liver that help the body metabolize many medications and other substances. CYP1A2, CYP2C9, CYP3A4, CYP2C19, and CYP2D6 are examples of enzymes involved in processing commonly used medications.
THC and CBD are also metabolized through several of these pathways and can influence the activity of certain CYP enzymes. If an enzyme’s activity increases or decreases, it can potentially change how quickly a medication is broken down and therefore how much of that medication is present in the body.
Common CYP450 pathways involved in potential cannabis-drug interactions. This chart provides examples and is not a complete list of possible interactions. The clinical significance of an interaction can vary depending on the medication, cannabis product, dose, route of administration, and individual patient.
Not every potential interaction will cause a clinically meaningful problem. However, interactions can be particularly important with medications that need to remain within a carefully controlled concentration range.
A 2024 systematic review examining real-world reports of cannabinoid interactions identified documented interactions involving several medications with narrow therapeutic ranges. Warfarin, valproate, tacrolimus, and sirolimus were among the more frequently reported examples.
2. Cannabis and Medication Side Effects Can Add Together
Not every interaction involves liver enzymes. Sometimes cannabis and another medication can produce similar effects, making those effects more pronounced when they’re used together.
Depending on the dose and individual response, THC may cause effects such as:
- Drowsiness
- Dizziness
- Impaired concentration
- Slower reaction time
- Confusion or disorientation in some individuals
Combining THC with another medication or substance that causes similar effects may increase the potential for drowsiness, dizziness, or impairment. This can be particularly important for driving, falls, workplace safety, and activities requiring concentration or coordination.
Which Medications May Need Extra Attention?
There isn’t one simple list of medications that are either “safe” or “unsafe” to combine with cannabis.
Health Canada identifies several medication and substance categories that may interact with cannabis, including:
- Blood thinners
- Anti-seizure medications
- Antidepressants and anti-anxiety medications
- Antipsychotics and mood stabilizers
- Sleeping medications and tranquilizers
- Some pain medications
- Immunosuppressants
- Certain heart medications
- Certain antibiotics and antifungal medications
- Some stomach-acid medications
- Antiretroviral medications
- Some allergy and cold medications
- Natural health products such as St. John’s wort
- Alcohol and grapefruit products
Being on one of these medications does not automatically mean you cannot use medical cannabis. The potential significance depends on factors such as the medication and dose, your health history, THC and CBD exposure, cannabis format, frequency of use, and other individual factors.
What Should You Tell Your Medical Cannabis Physician?
Before your Greenleaf assessment or follow-up, have a current list of:
💊 Prescription medications: Include the name, dose, and how often you take each medication.
🛒 Over-the-counter medications: Include occasional medications such as sleep aids, allergy medications, and pain relievers.
🌿 Vitamins and supplements: Include herbal remedies and natural health products.
🍃 Cannabis products: If you’re already using cannabis, include the THC and CBD content, format, approximate dose, frequency, and timing when possible.
🩺 Relevant medical conditions: Your overall medical history can influence whether a potential interaction is clinically important.
Already Using Cannabis? Tell Your Healthcare Provider
If you started using cannabis before seeking medical guidance, be open about what you’re currently using.
Tell your healthcare provider what you’re using cannabis for, whether it’s helping, any unwanted effects you’ve experienced, and whether your use has changed over time.
Health Canada recommends monitoring factors such as cannabis dose, product, symptoms, side effects, timing, and other medications and sharing that information with your healthcare provider.
Signs It May Be Time for a Medication Review
Talk with your healthcare provider or pharmacist if you notice:
- New or excessive drowsiness
- Increased dizziness or balance problems
- Confusion or difficulty concentrating
- Unusual bruising or bleeding
- Faintness or noticeable changes in heart rate
- Changes in seizure control
- Significant changes in mood or anxiety
- A medication suddenly seeming stronger or weaker than usual
Side effects can have many causes, so experiencing one doesn’t necessarily mean a drug interaction is occurring.
Seek urgent medical attention for severe symptoms such as chest pain, difficulty breathing, loss of consciousness, seizure, severe confusion, or signs of a serious allergic reaction.
Don’t Adjust Your Prescription Medication on Your Own
If you’re concerned about an interaction, don’t stop or change a prescribed medication without speaking with the healthcare provider responsible for it.
Taking cannabis and a medication several hours apart also doesn’t necessarily prevent an interaction. Some interactions involve how substances are metabolized and may extend beyond the period when you noticeably feel the effects of cannabis.
Your healthcare provider or pharmacist can determine whether additional monitoring or changes may be appropriate.
Why Follow-Up Matters
Medication review shouldn’t end after your first medical cannabis appointment.
Your health, medications, symptoms, and response to cannabis can all change over time. If you start or stop a medication, change a dose, experience new side effects, or notice cannabis affecting you differently, let your healthcare team know.
Greenleaf Medical Clinic provides ongoing medical cannabis care and follow-up so these changes can be considered as part of your overall plan.
Medical Cannabis Care Should Consider the Whole Patient
Medical cannabis doesn’t exist separately from the rest of your healthcare.
Your medications, diagnoses, symptoms, previous treatments, cannabis experience, potential risks, and healthcare goals all contribute to determining whether medical cannabis may be appropriate and how it should be approached.
At Greenleaf Medical Clinic, our physicians provide evidence-informed medical cannabis assessments and ongoing care. When medical cannabis is appropriate, patients receive individualized guidance around dosing, product formats, administration, and ongoing use.
Interested in becoming a Greenleaf patient?
🌿 For Patients: Talk to your doctor, specialist, or nurse practitioner about whether a referral to Greenleaf Medical Clinic may be appropriate, or start the process here.
🩺 For Healthcare Providers: Refer a patient for a medical cannabis assessment.
Medical disclaimer: This article provides general educational information and is not a substitute for diagnosis, medication review, or individualized medical advice. Do not start, stop, or change a medication or cannabis product without consulting an appropriate healthcare professional.
References
- Systematic review of drug-drug interactions of delta-9-tetrahydrocannabinol, cannabidiol, and Cannabis. Frontiers in Pharmacology, 2024.
- Cannabis for medical purposes: General information. Health Canada.
- Effects of cannabidiol and delta-9-tetrahydrocannabinol on cytochrome P450 enzymes: a systematic review. Drug Metabolism Reviews, 2024.
