CBD Drug Interactions: Which Medicines Need Caution

Why CBD interacts with medicines

Most medicines are cleared by a family of liver enzymes called cytochrome P450 (CYP450). CBD uses the same enzymes. It is metabolised mainly by CYP3A4 and CYP2C19, and in lab studies it inhibits CYP2C19, CYP2D6 and CYP2C9, as the 2021 review by Balachandran and colleagues (opens in a new tab) summarises.

That creates two kinds of interaction.

CBD changes the level of another drug. If CBD slows the enzyme that clears a medicine, more of that medicine stays in your blood. In a 2023 crossover study of 18 healthy adults (opens in a new tab), a single brownie with 640 mg CBD and 20 mg THC raised blood levels of omeprazole by 207%, losartan by 77%, midazolam by 56% and caffeine by 39%. It had no effect on dextromethorphan, the test drug for CYP2D6.

Another drug changes the level of CBD. The antibiotic rifampicin cut blood levels of CBD by about 30% and of its active breakdown product 7-OH-CBD by about 60%, according to the Epidyolex product information (opens in a new tab). The same document says carbamazepine, enzalutamide, mitotane and St John's wort may do the same.

There is a third route that has nothing to do with enzymes. CBD can cause drowsiness, lightheadedness, nausea and diarrhoea, and Harvard Health (2021) (opens in a new tab) notes that taking it with medicines that share those side effects can add them together.

Which drugs interact with CBD?

This table lists the medicine groups where a source reports an interaction or predicts one. "Predicted" means the source bases it on lab data or enzyme overlap, not on a human study. A group missing from the table has not been shown to be free of interactions.

Anti-seizure medicines

This is the best-studied area, because prescription CBD (Epidyolex) is used alongside other epilepsy drugs. In a healthy-volunteer study, CBD raised N-desmethylclobazam, the active metabolite of clobazam, 3- to 4-fold, and taking the two together raised the rate of sleepiness compared with placebo (Epidyolex SmPC (opens in a new tab)). With valproate, blood levels didn't change much, but liver enzyme rises became more common. The mechanism is unknown.

Warfarin

The main human evidence is a 2017 case report (opens in a new tab) of a 44-year-old man in an epilepsy programme. As his CBD dose was raised from a starting 5 mg/kg a day, his INR (the blood test that tracks warfarin's effect) rose, and his doctor cut his warfarin dose by about 30%. He had no bleeding complications. The authors suggest INR monitoring when cannabinoids are started or increased in people on warfarin.

Transplant drugs

A 2019 case report (opens in a new tab) describes a participant in a CBD epilepsy trial whose dose-adjusted tacrolimus levels rose about 3-fold while taking 2,000 to 2,900 mg of CBD a day. In a healthy-volunteer study, CBD raised exposure to everolimus about 2.5-fold, and the Epidyolex SmPC (opens in a new tab) says sirolimus, tacrolimus and digoxin levels may also rise.

Sedating medicines and alcohol

The BNF (opens in a new tab) lists alcohol, alprazolam, amitriptyline, baclofen and many other sedating drugs as interactions with cannabidiol, because both act on the central nervous system and can affect your ability to perform skilled tasks. The Epidyolex SmPC (opens in a new tab) says alcohol and other CNS depressants can add to CBD's sleepiness. If you drive, our guide to driving while taking CBD and sedating medicines covers the rules, and there is a separate page on CBD and muscle relaxants.

Antidepressants

Lab data suggest CBD could raise levels of some antidepressants through CYP2D6 and CYP2C19 (Balachandran 2021 (opens in a new tab)). In the 2023 human study (opens in a new tab), a single dose of 640 mg CBD with 20 mg THC did not inhibit CYP2D6, but it did inhibit CYP2C19. In a 2025 trial run by the FDA, 19 healthy adults took 5 mg/kg of CBD a day (about 350 mg for a 70 kg adult). After 7 days of CBD, blood exposure to a single 20 mg dose of the antidepressant citalopram was about 43% higher than with citalopram alone (ClinicalTrials.gov results, posted 2025 (opens in a new tab)). In the same trial, morphine exposure was about 12% higher after 7 days of CBD, a difference that was not statistically significant (opens in a new tab). More on this in our article on CBD and antidepressant safety limits.

Narrow therapeutic index medicines

Some medicines work only within a tight band of blood levels. Warfarin is one: its dose varies a lot between people and needs frequent INR checks (opens in a new tab). Anything that nudges those levels up or down matters more for these drugs than for most others.

Researchers at Penn State College of Medicine started from the prescribing information of approved cannabinoid medicines and built a list of 57 narrow-therapeutic-index medicines that cannabinoid use could affect, and a wider list of 139 medicines with a potential interaction (opens in a new tab) with prescription, over-the-counter or illicit cannabinoids (Kocis and Vrana, 2020). Harvard Health notes (opens in a new tab) that some of these are affected only by THC, not by CBD-only products. The Epidyolex SmPC (opens in a new tab) advises doctors to consider lowering the dose of narrow-index medicines broken down by CYP2C19.

Is the grapefruit warning a good rule for CBD?

Only partly. You'll see the tip online that if your medicine's leaflet warns against grapefruit, you should ask about CBD too. The overlap is real: grapefruit juice blocks CYP3A4 in the gut, according to the FDA (opens in a new tab), and CBD is itself metabolised by CYP3A4.

Even for CYP3A4 the human results differ. The Epidyolex SmPC (opens in a new tab) reports a study in which CBD did not change exposure to midazolam, the standard test drug for CYP3A4. In the 2023 study (opens in a new tab), one dose of 640 mg CBD with 20 mg THC raised midazolam by 56%.

The bigger gap is the other enzymes. The strongest effects in the 2023 study were on CYP2C19 and CYP2C9, and the documented clobazam and warfarin interactions mainly involve those two (SmPC (opens in a new tab), Grayson 2017 (opens in a new tab)). Grapefruit warnings don't cover those enzymes, so a leaflet without one doesn't rule anything out. A leaflet with one is a good reason to ask your pharmacist.

Do CBD drug interactions depend on the dose?

The main human findings come from these doses:

• Epidyolex trials: 10 to 25 mg per kg a day (SmPC (opens in a new tab)), or 700 to 1,750 mg a day for a 70 kg adult
• Enzyme study: a single dose of 640 mg CBD (Bansal 2023 (opens in a new tab))
• Tacrolimus case: 2,000 to 2,900 mg a day (Leino 2019 (opens in a new tab))
• Warfarin case: started at 5 mg per kg a day and increased (Grayson 2017 (opens in a new tab))

The 2021 review points out that interactions predicted from lab data may depend on concentration and may need very high levels of CBD to occur (Balachandran 2021 (opens in a new tab)).

The 2023 enzyme study also used a computer model to predict what lower doses would do (Bansal 2023 (opens in a new tab)). It predicted that 20 mg CBD twice a day for 7 days would raise levels of drugs cleared by CYP2C19, CYP3A and CYP1A2 by 25% or more, the cut-off the study used for an interaction. The lowest dose it predicted to cause no interaction with the test drugs was 5 mg twice a day. These are model predictions, not results measured in people. We found no clinical interaction study at food-supplement doses.

Can CBD affect liver tests?

Yes, in some people. Prescription CBD can cause dose-related rises in the liver enzymes ALT and AST. In controlled trials, 12% of patients on CBD had ALT above 3 times the upper limit of normal, compared with under 1% on placebo, and most of those with raised ALT were also taking valproate (Epidyolex SmPC (opens in a new tab)).

Lower doses have shown it too. In a 2025 randomised trial run by the FDA (opens in a new tab), 201 healthy adults took 5 mg/kg of CBD a day (about 350 mg for a 70 kg adult) or placebo for 28 days. Liver enzymes rose above 3 times normal in 8 of 151 people on CBD (an estimated 5.6%) and in none on placebo.

If you're having blood tests or take another medicine that affects the liver, tell your doctor about CBD so the results can be read correctly. Our guide to CBD side effects and liver questions goes into more detail.

Does topical CBD interact with medications?

Less is known. Harvard Health (opens in a new tab) says creams and lotions may not get enough CBD into the blood to interact with other medicines, but there is very little data on how much is absorbed. Inhaled CBD reaches the blood fastest, and edibles more slowly. Our comparison of CBD capsules and oil covers how the form affects absorption.

Is there a CBD drug interaction checker?

There are lists, but none that can check a food supplement for you:

• The BNF cannabidiol interactions page (opens in a new tab), the interaction list in the UK's national formulary
• The Penn State cannabinoid interaction lists (opens in a new tab), a list of 57 narrow-index medicines and a wider list of 139
• General checkers such as Medscape, where searches for CBD interactions typically yield few results (opens in a new tab), according to the 2021 review

These lists are built on the prescribing information for prescription cannabinoids. They tell you which medicines to ask about. They can't tell you how a particular food supplement combines with your own medicines, which is why a pharmacist is the better check.

Who should not take CBD with medication?

The EU and UK food regulators both draw a line here:

• EFSA (EU): safety can't be established for people on medication, under-25s, or pregnant and breastfeeding women (February 2026 (opens in a new tab)).
• FSA (UK): CBD is not recommended, unless under medical direction, for people taking any medication, children, people trying to conceive, or those pregnant or breastfeeding (FSA consumer advice (opens in a new tab)).

Harvard Health also advises telling your doctor about CBD if you have liver or kidney disease, epilepsy, heart problems or a weakened immune system (opens in a new tab). The same article notes that older adults are more prone to drug interactions because they often take several medicines. For the legal status of CBD food products, see CBD rules in Europe.

What to bring to your pharmacist or doctor

A useful conversation needs details. "Is CBD safe?" can't be answered without your medicine list. Bring:

1. The exact CBD product name and the CBD amount per serving and per day
2. The full ingredient list, including anything else in the product
3. The product's lab report, if it has one
4. Every prescription medicine, with doses
5. Over-the-counter products you use even occasionally, such as sleep aids, antihistamines and painkillers
6. Other supplements and herbal products, for example St John's wort
7. How much alcohol you drink
8. Any planned surgery or blood tests

If you take vitamins or other supplements alongside CBD, our page on combining CBD with other supplements covers that separately.

Don't start several new products at once, and don't change a prescribed medicine because of anything you read here. If you notice unusual drowsiness, dizziness, diarrhoea or other new symptoms after adding CBD, or unusual bruising or bleeding if you take warfarin, contact your doctor or pharmacist.

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