Medical Cannabis Drug Interactions

Q: Can I take CBD with blood thinners?

CBD inhibits the CYP2C9 enzyme, which metabolises warfarin and other anticoagulants. This can cause warfarin levels to rise significantly, increasing bleeding risk. If you take warfarin or any anticoagulant, you must inform your prescribing doctor and have your INR monitored more frequently when starting or changing your cannabis dose.

Q: Does THC interact with antidepressants?

THC and some antidepressants (particularly SSRIs and SNRIs) act on similar serotonin pathways. The risk of serotonin syndrome from cannabis alone is low, but combining high-THC products with certain antidepressants warrants caution. Always disclose your antidepressant to your cannabis prescriber.

Q: Is medical cannabis safe with blood pressure medication?

Cannabis can temporarily raise heart rate (particularly THC) and has variable effects on blood pressure. Beta-blockers may partially blunt the tachycardia caused by THC. CBD can mildly lower blood pressure. For most patients on stable antihypertensives the interaction is modest, but dose adjustments may be needed.

Q: Should I tell my GP about my cannabis prescription?

Yes — absolutely. Your GP needs to know about all medications you take, including prescribed cannabis, to safely manage any other conditions and avoid dangerous drug combinations. Many clinics will write to your GP automatically, but you should proactively confirm this has happened.

Q: What is the safest way to start medical cannabis with existing medications?

Start at the lowest effective dose and increase slowly (start low, go slow). Bring a complete medication list to your cannabis clinic consultation. Ask your prescriber specifically about each medication you take. Arrange closer monitoring with your GP for high-risk combinations such as anticoagulants or antiepileptics.

A complete guide for UK patients on how cannabis — THC and CBD — can interact with prescription medications. Last reviewed May 2026.

Why Drug Interactions Matter for UK Patients

Medical cannabis is now prescribed to tens of thousands of patients in the UK, many of whom take other prescription medications for chronic conditions. Cannabis is not simply a benign supplement — both THC and CBD are pharmacologically active compounds that can meaningfully affect how other drugs are processed by your body.

Understanding potential interactions before starting treatment helps you and your clinical team monitor for problems early, adjust doses where needed, and stay safe. This guide covers the most clinically relevant interactions known at time of publication. It is not exhaustive — always discuss your complete medication list with your prescribing specialist.

How Cannabis Affects Drug Metabolism

The majority of cannabis-drug interactions occur because cannabinoids — principally CBD but also THC — interfere with a family of liver enzymes called cytochrome P450 (CYP450). These enzymes metabolise a huge proportion of prescription drugs. When an enzyme is inhibited, it processes its target drug more slowly, causing that drug to accumulate to higher-than-expected blood levels.

CYP3A4 — metabolises roughly 50% of all prescription medicines, including many statins, immunosuppressants, calcium channel blockers, and benzodiazepines. Both THC and CBD inhibit this enzyme, with CBD being the more potent inhibitor.
CYP2C9 — metabolises warfarin, many NSAIDs, and some antidiabetic drugs. CBD is a significant CYP2C9 inhibitor; even moderate CBD doses can raise warfarin levels meaningfully.
CYP2C19 — metabolises some antidepressants (citalopram, escitalopram), proton pump inhibitors, and certain antiepileptics. CBD inhibits this enzyme at higher doses.

CBD is generally a stronger CYP450 inhibitor than THC. At the doses used therapeutically (above approximately 300 mg/day for CBD), these interactions can be clinically significant. Even lower doses may matter for drugs with narrow therapeutic windows such as warfarin or ciclosporin.

High-Risk Combinations Monitor Closely

These combinations require close medical supervision

Warfarin & Anticoagulants (rivaroxaban, apixaban, heparin)

CBD inhibits CYP2C9, the primary enzyme that breaks down warfarin. This can cause warfarin blood levels to rise significantly, substantially increasing the risk of serious bleeding. Patients on anticoagulants who start medical cannabis must have more frequent INR monitoring. Dose adjustments to anticoagulant therapy are often needed. Never start cannabis alongside anticoagulants without informing your prescriber.

Antiepileptics — Clobazam & Valproate

CBD is licensed in the UK as Epidyolex for certain epilepsy syndromes, but even at lower doses CBD significantly raises blood levels of clobazam (and its active metabolite norclobazam) by inhibiting CYP2C19 and CYP3A4. Elevated clobazam causes sedation and ataxia. Valproate levels can also be affected. If you take antiepileptic drugs, your neurologist needs to be involved in any decision to start medical cannabis.

Immunosuppressants — Ciclosporin & Tacrolimus

These are drugs with very narrow therapeutic windows used after organ transplants and in autoimmune conditions. Both are CYP3A4 substrates. CBD and THC can raise ciclosporin and tacrolimus levels considerably, increasing the risk of toxicity (kidney damage, infections). Any transplant patient must consult their transplant team before starting any cannabis product.

Sedatives & Benzodiazepines (diazepam, lorazepam, zopiclone)

THC and CBD both have CNS-depressant properties. Combining cannabis with benzodiazepines or other sedatives creates additive central nervous system depression — excessive sedation, respiratory depression, and impaired coordination. This is particularly dangerous in elderly patients or at higher doses. Doses of both agents may need to be reduced.

Moderate Caution Be Aware

Monitor for changes; inform your prescriber

SSRIs & SNRIs — Antidepressants (sertraline, fluoxetine, venlafaxine)

The risk of serotonin syndrome from cannabis alone is low, but several SSRIs are metabolised by CYP2D6 and CYP2C19, both of which are inhibited by CBD to varying degrees, meaning antidepressant levels may rise. Additionally, cannabis can worsen anxiety or low mood in some patients. Monitor mood carefully when starting cannabis alongside antidepressants and report any unusual symptoms to your GP.

Beta-Blockers (propranolol, atenolol, bisoprolol)

THC typically causes an increase in heart rate (tachycardia). Beta-blockers reduce heart rate and may partially offset this effect. However, CBD can mildly lower blood pressure, and some CYP2D6-metabolised beta-blockers (e.g. metoprolol, propranolol) may accumulate if CBD is taken concurrently. Patients should monitor blood pressure and heart rate when starting medical cannabis.

Statins (atorvastatin, simvastatin)

Atorvastatin and simvastatin are primarily metabolised by CYP3A4. CBD inhibition of this enzyme can increase statin blood levels, raising the risk of statin-related side effects including muscle pain (myopathy) and, rarely, rhabdomyolysis. Pravastatin and rosuvastatin are less affected as they are not CYP3A4-dependent. Inform your prescriber if you take a statin.

NSAIDs (ibuprofen, naproxen)

NSAIDs are generally considered lower risk in combination with cannabis. However, both cannabis and NSAIDs can affect gastric mucosa, and the CYP2C9-mediated metabolism of ibuprofen may be mildly affected by CBD. For most patients this is not clinically significant, but those with GI conditions or on high NSAID doses should exercise caution.

Generally Safe Low Risk

These combinations are generally considered low risk

Paracetamol (acetaminophen)

Paracetamol is not metabolised by CYP2C9 or CYP3A4 in significant quantities and has no known clinically meaningful pharmacokinetic interaction with cannabinoids. It remains the recommended first-line analgesic for patients on medical cannabis who need occasional pain relief.

Most Antibiotics (amoxicillin, doxycycline, trimethoprim)

Common antibiotics are generally metabolised through pathways not significantly affected by cannabinoids. Short antibiotic courses during medical cannabis treatment are typically safe without dose adjustment. Clarithromycin and erythromycin are exceptions as they interact with CYP3A4, but the interaction is usually short-lived.

Vitamin D & Common Supplements

Vitamin D3, magnesium, zinc, and most common dietary supplements have no significant pharmacokinetic interaction with cannabis. At normal supplemental doses this is not clinically significant.

Practical Advice for UK Patients

Tell your GP. Your general practitioner needs to know about every prescription, including medical cannabis. Ask your cannabis clinic to write to your GP directly — many do this as standard practice, but confirm it has happened.
Bring your medication list to every clinic appointment. Include prescription medicines, over-the-counter products, herbal supplements, and vitamins. Drug interactions can involve unexpected substances.
Start low and go slow. The principle of starting at the lowest effective dose and titrating upward slowly reduces the risk of dose-dependent interactions and allows time to observe any changes in existing medication levels or effects.
Arrange closer monitoring when needed. If you take warfarin, anticoagulants, antiepileptics, or immunosuppressants, actively arrange additional blood level monitoring with your GP or specialist when starting or changing your cannabis dose.
Do not stop other medications without advice. Cannabis can reduce symptoms that other medications are controlling. Do not adjust existing medications based on perceived improvement without consulting your prescriber.
Report new symptoms promptly. Unexpected excessive sedation, unusual bleeding, muscle pain, mood changes, or any new symptom after starting cannabis should be reported to both your cannabis clinic and your GP without delay.

Important Medical Disclaimer

This guide is for general patient education only and does not constitute medical advice. The information provided is based on published pharmacological data and clinical guidance current as of May 2026, but the evidence base for cannabis-drug interactions is still evolving.

This page cannot capture every possible interaction for every individual patient situation. Drug interactions depend on dose, route of administration, individual genetic variation in CYP enzyme activity, and many other factors unique to each patient.

Always consult your prescribing doctor, your GP, and a clinical pharmacist before starting or changing your medical cannabis prescription if you take other medications. Do not make changes to any existing medication based solely on information found on this website.

If you experience any unexpected side effects or changes in how your other medications feel, seek medical advice promptly.

Frequently Asked Questions

Can I take CBD with blood thinners? +

Does THC interact with antidepressants? +

Is medical cannabis safe with blood pressure medication? +

Should I tell my GP about my cannabis prescription? +

What is the safest way to start medical cannabis with existing medications? +

Further reading: Browse our Patient Guides for more information on conditions, costs, and prescriptions — or visit Conditions to see what medical cannabis is prescribed for in the UK. For cannabis-specific terminology see our full FAQ.