CBD and medication: the grapefruit effect in numbers

Most medicines are cleared by enzymes, mainly in the liver and the gut wall. The best-known family is the cytochromes P450, written CYP. When a substance slows one of these enzymes, the medicine it should break down builds up. Its effect, and its side effects, can then increase.

The CYP3A4 enzyme matters a great deal: New Zealand's medicines agency, Medsafe, states that it handles more than 50% of medicines. A 2023 review in Frontiers in Psychiatry lists the enzymes CBD slows: CYP2C9, CYP2C19, CYP2D6 and CYP3A, with a possible effect on CYP1A2, CYP2B6 and CYP2C8. CBD may also slow two conjugation enzymes, UGT1A9 and UGT2B7. Many of these results come from laboratory tests on cells or isolated enzymes.

Our article on CBD and the liver explains the role of these enzymes and liver monitoring under Epidyolex.

Grapefruit is the usual example because its effect is well documented. A 2013 review in the CMAJ counted more than 85 medicines that interact, or are expected to interact, with the fruit. For 43 of them, the interaction can have serious consequences.

The mechanism is specific. Compounds in the fruit, furanocoumarins, irreversibly inactivate CYP3A4 in the small intestine. One whole grapefruit or 200 ml of juice is enough. The effect is greatest when the fruit is eaten within 4 hours before the medicine; about 25% of it remains after 24 hours. A medicine given intravenously is not affected, because it does not pass through the gut.

The analogy holds for the principle: a seemingly harmless substance changes the real dose of a medicine. It does not hold for the enzymes involved, or for the amounts.

The best data come from the approval file of Epidyolex, the only CBD-based medicine. Its summary of product characteristics, published by the European Medicines Agency, describes studies in healthy volunteers. CBD was given at 7.5 to 12.5 mg per kilo twice a day, or about 1,000 to 1,750 mg a day for 70 kg.

Four results deserve attention:

• Clobazam, an antiepileptic. The level of its active metabolite, N-desmethylclobazam, is 3 to 4 times higher, probably through inhibition of CYP2C19. Drowsiness affected 43% of patients on CBD and clobazam, against 14% on CBD without clobazam.
• Everolimus, an immunosuppressant. Its exposure rises about 2.5 fold. The agency attributes this to a gut transporter, P-gp, and names tacrolimus, sirolimus and digoxin as exposed to the same risk.
• Caffeine. Its total exposure rises by 95%, which makes CBD a weak inhibitor of CYP1A2.
• Midazolam, the medicine used as a marker of CYP3A4. CBD did not change its exposure. At these doses, CBD does not behave like grapefruit towards this enzyme.

The same document notes that combining CBD with valproate increases how often liver enzymes rise. A case published in 2017 describes a 44-year-old patient followed for epilepsy who was also taking warfarin, an anticoagulant. His INR, the clotting marker monitored on this treatment, went from 2.22 to 6.86 when CBD reached 10 mg per kilo per day. His warfarin dose had to be cut by about 30%.

All these measurements concern medicinal doses. Epidyolex is prescribed at 5 to 20 mg per kilo per day, up to 25 mg in one indication, or 350 to 1,400 mg a day for 70 kg. The 2023 review considers strong interactions likely with high-dose oral CBD, around 700 mg. It also cites a rise in citalopram, an antidepressant, with CBD products dosed at 200 to 800 mg a day.

At the doses of products sold to the public, studies are lacking. No data does not mean no risk, and the agencies say so clearly. EFSA set on 9 February 2026 a provisional level of 2 mg a day for a 70 kg adult, but could not establish safety for people taking medicines. The UK Food Standards Agency advises people on medication against CBD. Our article on how much CBD per day compares these benchmarks.

In France, swallowing CBD also raises a legal question: CBD foods and supplements must be withdrawn from sale, as our overview of CBD regulation in 2026 explains.

The right person to ask is the pharmacist or doctor who knows your treatment. They can check each molecule, which no general list can do. To prepare that conversation:

1. List all your medicines, including over-the-counter ones, supplements and St John's wort, which also acts on these enzymes.
2. Spot the narrow-margin ones, where a small change in concentration matters: anticoagulants such as warfarin, immunosuppressants such as tacrolimus or everolimus, antiepileptics, digoxin.
3. Never change on your own the dose of a prescribed medicine, whether to "make room" for CBD or the other way round.
4. Mention CBD before surgery, like any product you use. Our article on CBD before anaesthesia reviews the question.
5. Allow for drowsiness. The Epidyolex summary states that alcohol and other nervous system depressants can strengthen it.

The side effects of CBD itself are covered in our article on its side effects, and the situation of older people, who often take several medicines, in our article on CBD for seniors.