One of the questions we're most often asked is: can I take CBD with the medication I'm already on? The honest answer is usually "probably fine, but check". Here's what's behind the "probably".
What about medications that make you drowsy?
CBD can make you sleepy on its own, and so can sedating antihistamines, sleeping tablets, codeine, some antidepressants and alcohol. Pair CBD with any of those and the drowsiness can add up.
Apart from drowsiness, how else might CBD interact with my medication?
Through the liver. Cannabidiol (CBD) is broken down by a family of liver enzymes called cytochrome P450, and while it's being processed it slows some of them down. A lot of prescription medicines are cleared by the same enzymes. If CBD is keeping them busy, a medicine can be cleared more slowly and build up to a higher level in your blood than your prescriber intended.
That's the whole mechanism. The interactions that CBD might have are based on which medicines rely on those enzymes.
A pharmacist can check a specific medicine against CBD in a minute.
Remember the grapefruit rule
An unusual rule of thumb: if a medicine's leaflet tells you to avoid grapefruit, ask before adding CBD too. Grapefruit blocks one of those same liver enzymes, CYP3A4, which is why the NHS pages for sertraline, amlodipine, diazepam and simvastatin all warn against it. CBD works on the same family of enzymes.
The match isn't exact. The clearest evidence for CBD is on 2 neighbouring enzymes rather than the grapefruit one, so treat it as worth checking rather than as a strict rule. A grapefruit warning suggests a medicine is sensitive to liver enzymes, and that's exactly the kind of medicine to ask about.
Which interactions have been seen in patients?
Fair warning: this bit gets technical. It'll matter most if one of these conditions or medicines is already familiar to you.
Almost everything we know from patients comes from Epidyolex, the licensed CBD medicine prescribed for rare, severe seizure disorders. Its trials measured what happened when it was added to other drugs. 3 interactions stand out.
Most CBD interaction lists are long on theory and short on patients.
Warfarin. CBD slows CYP2C9, the enzyme that clears warfarin, so Epidyolex's manufacturer expects warfarin levels to rise. A published case report followed a man on a stable warfarin dose who started Epidyolex: his INR, the number that tracks how thin the blood is, climbed, and his warfarin dose ended up cut by about 30%. A 2024 systematic review found 7 published warfarin cases with cannabinoid products, 2 of them CBD oil, and 5 of the 7 needed a dose change.
Clobazam. The best-studied of all. Clobazam is a benzodiazepine used for seizures, and CBD raises the level of its active by-product 3- to 4-fold, with more sleepiness to match. In a study of 13 children taking both, 10 had their clobazam dose lowered.
Valproate. Another seizure medicine, and the one the BNF marks as severe: people taking it alongside CBD had raised liver enzymes more often.
Now the honest bit. All of this comes from Epidyolex at clinical doses, typically 10 to 20mg per kilo of body weight a day. For a 70kg adult that's 700 to 1,400mg.
Our oil's serving is 25mg and the FSA's suggested limit is 10mg a day. We can't tell you how to read that gap. The clinical dose is somewhere between 28 and 56 times our serving. The mechanism doesn't change with the amount; whether the size of the effect does hasn't been measured at supplement servings.
Why aren't ibuprofen and paracetamol on this list?
You'll see them on other sites, usually alongside levothyroxine. Ibuprofen rests on a test-tube finding that CBD slows an enzyme they share; paracetamol on a theory about the liver; levothyroxine on nothing we could find. None of the 3 appears in the BNF's list for cannabidiol. Theoretical isn't the same as impossible, but we'd rather give you 3 interactions with patients behind them than 12 with a shrug.
Who should I ask?
A pharmacist, first. Your GP can check for interactions, but most can't say much about CBD itself, and Ruby Deevoy, the UK's only dedicated CBD columnist, has explained why. A pharmacist can look a specific medicine up in a minute and you don't need an appointment. Go in with the medicine's name and the amount of CBD you're planning to take, in milligrams rather than drops.
We can't make that call for you. The FSA's own guidance is that CBD isn't recommended for anyone taking medication without checking with a doctor or pharmacist first.
CBD is a food supplement, not a medicine. This article is for informational purposes only and does not constitute medical advice. If you're taking medication, or you're pregnant or breastfeeding, consult your GP or pharmacist before using CBD.
References
- Epidyolex 100 mg/ml oral solution: Summary of Product Characteristics, sections 4.4, 4.5 and 5.2. electronic medicines compendium (emc).
- Grayson, L., Vines, B., Nichol, K. & Szaflarski, J.P. (2018). An interaction between warfarin and cannabidiol, a case report. Epilepsy & Behavior Case Reports, 9, 10–11.
- Nachnani, R., et al. (2024). Systematic review of drug-drug interactions of delta-9-tetrahydrocannabinol, cannabidiol, and Cannabis. Frontiers in Pharmacology, 15, 1282831.
- Geffrey, A.L., Pollack, S.F., Bruno, P.L. & Thiele, E.A. (2015). Drug-drug interaction between clobazam and cannabidiol in children with refractory epilepsy. Epilepsia, 56(8), 1246–1251.
- Kocis, P.T. & Vrana, K.E. (2020). Delta-9-Tetrahydrocannabinol and Cannabidiol Drug-Drug Interactions. Medical Cannabis and Cannabinoids, 3(1).
- British National Formulary (NICE). Cannabidiol: interactions. 181 entries as listed on 22 September 2026.
- Food Standards Agency (updated October 2023). Cannabidiol (CBD): consumer guidance. gov.uk.
- NHS medicines A to Z: sertraline, amlodipine, diazepam and simvastatin (grapefruit warnings).