Cannabis and Your Medications: Questions to Ask Your Clinician Before You Buy
If you take prescription medication, adding cannabis is worth a conversation with a clinician or pharmacist first. THC and CBD are processed by some of the same liver enzymes that handle many prescriptions, which is why interactions are an active area of research. This guide summarizes what published reviews report and gives you a list of questions to bring to your appointment. It is general education, not medical advice. Do not start, stop or change any medication on your own.
Quick takeaways
THC and CBD are metabolized by liver enzymes (CYP3A4, CYP2C9 and CYP2C19) that also process most prescription medications.
A 2024 systematic review found that warfarin, valproate, tacrolimus and sirolimus were among the medications most widely reported in cannabinoid interactions.
The overall evidence is limited. A 2022 review identified only 19 interaction pairs in humans and stressed the lack of evidence.
The best step is to tell your prescriber and your pharmacist exactly what cannabis products you use.
Why cannabis and medications can interact
Your liver uses a family of enzymes called cytochrome P450, or CYP enzymes, to break down many drugs. According to a 2024 systematic review in Frontiers in Pharmacology, the major cannabinoids THC and CBD are metabolized by the same enzymes that metabolize most prescription medications: CYP3A4, CYP2C9 and CYP2C19 (review). If cannabinoids change how quickly a medicine is cleared, the amount of that medicine in the blood can rise or fall. Cannabis can also add to the effects of other medicines, for example, those that cause drowsiness.
That matters most for medicines with a narrow therapeutic index, where a small change in blood level can make a big difference.
What published reviews report
2024 systematic review: the authors screened 4,600 reports and found 31 in which cannabinoids altered drug levels or produced adverse events. They involved 16 narrow-therapeutic-index medications. Interactions with warfarin, valproate, tacrolimus and sirolimus were the most widely reported. In 18 cases, clinicians found an unexpected blood level of the prescribed drug. The authors urge patients and healthcare providers to communicate openly about cannabis use (Frontiers in Pharmacology).
Blood thinners: a 2020 review of anticoagulant and antiplatelet drugs described case reports in which cannabis may inhibit the metabolism of warfarin, raising blood levels and the risk of bleeding. It also notes that CBD inhibits CYP2C19, an enzyme involved in activating clopidogrel, which could in theory reduce that drug's effect (review).
Seizure medicine: a clinical resource from the University of Mississippi notes that CBD inhibits CYP2C19 and can raise levels of an active metabolite of the seizure drug clobazam threefold (clinical resource).
2022 systematic review: this review identified 19 pairs of drug interactions with cannabis in humans and concluded that clinically relevant interactions appeared to have a low probability while the evidence remained limited (review).
Taken together, the picture is mixed. Interactions are plausible, and some are documented for particular medicines, but good-quality evidence is limited. That is exactly why it helps to ask before you start.
Questions to ask your clinician or pharmacist
I take this medication. Is it processed by CYP3A4, CYP2C9 or CYP2C19, and does that matter if I use cannabis?
Is it a medicine where a small change in blood level matters? Would I need extra monitoring or blood work?
Does the form change the risk: inhaled, an edible, or a product with only CBD?
Could cannabis add to drowsiness or dizziness from my other medicines?
What symptoms should I watch for, and when should I call you or seek urgent care?
Does the timing of my cannabis use relative to my medication doses matter?
Do my other health conditions change your advice?
Can you note my cannabis use in my record so every provider sees the full picture?
Is there an interaction checker you trust? The authors of the 2024 review mention a free tool from Penn State called CANN-DIR.
What to bring to the appointment
A list of every prescription, over-the-counter medicine and supplement you take, with doses.
The cannabis products you use or are considering: name, form, THC and CBD amounts. A photo of the label works well.
How often and when you use them, and any side effects you have noticed.
Your questions, written down, so nothing gets lost in a short visit.
Safety reminders
Do not stop, start or change any prescription without talking to the clinician who prescribes it.
CBD-only products are not automatically free of interaction concerns, because CBD is metabolized by the same enzymes.
Edibles act slowly and can last a long time, so read our guide to weed edibles dosing and onset time. Our article on weed carts covers inhaled products.
Do not drive or operate machinery if you feel impaired, and keep all cannabis products locked away from children and pets.
Medical cannabis at Best Stop: what we can and cannot do
Best Stop Dispensary at 3439B 14th St NW in Columbia Heights is a licensed medical cannabis dispensary serving eligible registered patients and caregivers. Our team can explain what is on a product label, such as THC and CBD amounts and package sizes. We cannot give medical advice or tell you how cannabis will interact with your medicines. That conversation belongs with your clinician or pharmacist. Registration is handled by the District's Alcoholic Beverage and Cannabis Administration (ABCA). Our get a card page and our guide to DC resident self-certification explain the process and how we can help you through it.
Frequently asked questions
Does CBD interact with medications?
It can. CBD is metabolized by the same liver enzymes as many prescription medicines, and the research above includes documented CBD interactions. Ask your pharmacist before combining CBD with a prescription.
Is it safe to use cannabis with blood thinners?
That is a question for your prescriber. Published case reports describe higher blood-thinning levels with warfarin and cannabis, so this is a conversation to have before you start, not after.
Where can I check for interactions?
Start with your pharmacist. The authors of the 2024 review also point to a free online tool from Penn State called CANN-DIR. A tool is a starting point, not a substitute for professional advice.
Should I tell my doctor that I use cannabis?
Yes. Your clinician can only account for what they know about. Share the products, amounts and timing.
This article is educational and is not medical or legal advice. Research on cannabis and drug interactions is still developing, and DC rules and product availability can change.
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