CBD side effects: liver, medicines and who should avoid it

Yes, CBD can cause side effects, though most are mild and linked to dose. The most common in clinical trials, mainly at prescription doses of 10–25 mg per kg a day, are sleepiness, diarrhoea, reduced appetite and tiredness. At higher doses CBD can raise liver enzymes, and it can change how some medicines work. Pregnant or breastfeeding women, people under 18 and anyone taking medication should avoid CBD or ask a doctor first.

Is CBD oil safe?

In clinical trials CBD was generally well tolerated, but it is not free of risk. Most of what we know comes from trials of a prescription epilepsy medicine, at doses far higher than a typical food supplement. Those trials show that side effects become more likely as the dose goes up, and that the liver and other medicines are the main concerns.

The World Health Organization's expert committee concluded in 2018 that CBD "is generally well tolerated with a good safety profile" and that in humans it "exhibits no effects indicative of any abuse or dependence potential" (WHO Expert Committee on Drug Dependence, 2018).

Food regulators have therefore set cautious reference amounts, well below the trial doses.

What are the common side effects of CBD oil?

The best data come from the controlled trials behind Epidyolex, the prescription CBD medicine approved in the EU for three rare forms of epilepsy that usually begin in childhood. At 10–25 mg of CBD per kg of body weight a day, the most common side effects were:

• Sleepiness or sedation (23%)
• Reduced appetite (21%)
• Diarrhoea (20%)
• Fever (16%), vomiting (12%) and tiredness (10%)

Rash and irritability were less frequent. Many effects depended on the dose and were more frequent with other epilepsy medicines. Sleepiness, for example, affected 43% of patients who also took clobazam, against 14% of those who did not (EMA, Epidyolex product information).

A 2020 review of 12 placebo-controlled trials (803 participants) came to a similar view. Outside the childhood epilepsy studies, diarrhoea was the only side effect clearly linked to CBD. The authors concluded that CBD "is well tolerated and has relatively few serious adverse effects", but that interactions with other medicines need careful monitoring (Chesney et al., 2020).

Do side effects happen at food-supplement doses?

This is the honest gap in the research. The large trials used medicine doses: for a 70 kg adult, 10 mg per kg a day is 700 mg of CBD a day. The reference amounts regulators have set for CBD eaten as food are far lower, and there are few good trials at those levels.

What we can say is that diarrhoea, weight loss and liver-enzyme rises became more frequent as the dose went up. The charts show how far apart food reference amounts and trial doses are.

Can CBD affect the liver?

Yes, and this is the side effect regulators take most seriously. CBD is broken down in the liver. At higher doses it can raise the liver enzymes ALT and AST, which doctors use as a warning sign of liver stress.

• At prescription doses: in the Epidyolex trials, 12% of patients had ALT above three times the normal limit, against under 1% on placebo. The risk was highest with the epilepsy medicine valproate (19–23%) and lowest in patients taking neither valproate nor clobazam (3%). About two-thirds of cases resolved when the dose was lowered or stopped, and about one-third resolved while treatment continued (EMA).
• In healthy adults: a 2025 placebo-controlled trial gave 5 mg per kg a day for 28 days, a dose chosen because some consumers take that much. 5.6% of the CBD group had liver enzymes above three times the normal limit, against none on placebo. Seven met the criteria for stopping because of possible drug-induced liver injury (Florian et al., 2025).
• In animal studies: liver changes were the most consistent finding, and EFSA used them as the basis for its provisional safe intake (EFSA, 2026).

Can CBD interact with medication?

Yes. CBD is broken down by the same liver enzymes as many medicines, so it can raise or lower their levels in the blood; in studies, for example, it raised levels of the active breakdown product of the epilepsy medicine clobazam and of the antidepressant citalopram (EMA; Clinical Pharmacology & Therapeutics, 2026). If you take any regular medicine, speak to your doctor or pharmacist first, and see our guide to CBD and medication for which medicines need most caution.

Who should avoid CBD?

More on CBD and children, drug tests and driving, and drowsiness in CBD oil and sleep.

What do EFSA and the UK FSA say about CBD?

In February 2026 the European Food Safety Authority (EFSA) updated its 2022 statement on CBD as a novel food. Its expert panel set a provisional safe intake of 0.0275 mg per kg of body weight a day, about 2 mg a day for a 70 kg adult (EFSA Journal, 2026). Three points are easy to miss:

1. It is a precautionary number. EFSA started from a dose linked to liver changes in 90-day animal studies (11 mg per kg a day) and divided it by 400, because so much data is missing. It isn't a dose at which side effects were seen in people.
2. It covers pure CBD only. It applies to food supplements with CBD at least 98% pure, without nanoparticles, from a production process judged safe. It doesn't directly assess full-spectrum or broad-spectrum hemp extracts.
3. It excludes vulnerable groups. EFSA concluded that safety can't be established for people under 25, pregnant or breastfeeding women, or people taking medication.

EFSA also listed what is still unknown: whether CBD builds up in the body with long-term use, how far the liver, reproductive and nervous-system effects seen in animals apply to people, and possible effects on thyroid hormones. No studies have yet looked at effects on the immune system.

The UK Food Standards Agency

Since October 2023 the FSA has advised healthy adults to limit CBD from food to 10 mg a day. It advises children under 18, people taking any medication, and those trying to conceive, pregnant or breastfeeding to avoid CBD unless under medical direction. It adds that "the more CBD you consume over your lifetime, the more likely you are to develop long-term adverse effects, like liver damage or thyroid issues" (FSA consumer advice; restated on the FSA blog, June 2026).

In September 2026 the FSA board recommended that ministers authorise the first CBD foods in England and Wales, with label warnings for under-18s, people who are pregnant, breastfeeding or trying to conceive, and people taking medication or who are immunosuppressed. Ministers make the final decision (FSA board paper, September 2026).

Is CBD safe to use long term?

Nobody knows yet. Most trials last weeks or months, and EFSA says it isn't known whether CBD builds up in the body over time. If you use CBD every day for months, use the lowest amount that suits you and mention it to your doctor at your next check-up.

Could the side effects come from the product rather than the CBD?

Sometimes, yes. A poorly made oil can contain much more THC than stated, a different amount of CBD than the label says, or contaminants. A study published in August 2026 analysed 186 CBD and related oils sold in Europe over about a decade (Binova et al., 2026).

• 49% (86 of 181) didn't contain the amount of CBD stated on the label.
• At the recommended dose, 39% (72 of 186) would exceed EFSA's acute reference dose for THC, 1 µg per kg of body weight. Some also contained other THC variants, and one "full-spectrum" oil contained HHC.
• Polycyclic aromatic hydrocarbons (PAHs), a group of contaminants, were found in every sample, and 54% were above the EU maximum for oil-based food supplements.

That is why it pays to buy directly from a producer that makes, tests and labels its own oils. At Formula Swiss every production batch is tested, the label shows CBD and THC to two decimal places, and we publish the certificate of analysis. If you need zero THC, choose a THC-free broad-spectrum oil.

Unexpected THC can cause effects people wrongly blame on CBD, such as dizziness, anxiety or a racing heart.

At Formula Swiss, every production batch is tested, and we publish the certificates of analysis (how we test). To compare THC-free and full-spectrum oils, see full-spectrum or broad-spectrum?

What should I do if I get side effects?

1. Lower the amount or stop. Most mild effects, such as sleepiness or loose stools, settle when you use less. Our CBD oil dosage guide shows how much CBD is in one drop of each strength.
2. If CBD makes you sleepy, don't drive.
3. Stop and see a doctor if you notice signs of liver problems, a rash, or a change in how a medicine works.
4. Report it. Tell the seller, and report suspected reactions to your national food, cosmetics or medicines authority.

Does CBD have side effects in pets?

Animals process CBD differently from people, so never give a pet an oil made for humans, and ask your vet first. Our guides to CBD for dogs and CBD for cats cover the animal studies and what to watch for.

See the latest batch results for our oils, including CBD and THC content, on our lab reports page.