CBD side effects: liver, drowsiness and drug interactions
CBD (cannabidiol) is well tolerated in most studies and has documented side effects: drowsiness, diarrhoea, reduced appetite and, in some people, raised liver enzymes. Their frequency depends mainly on the dose and on any medicines taken alongside. In February 2026 EFSA set a provisional safe level of about 2 mg a day for a 70 kg adult.
Key takeaways
- Drowsiness, diarrhoea and reduced appetite come first, and they are usually mild to moderate.
- An FDA trial published in 2025 gave healthy volunteers 5 mg/kg a day: 5.6% had liver enzymes above 3 times the normal limit.
- CBD slows CYP3A4 and CYP2C19, two liver enzymes that clear many medicines from the body.
- EFSA sees no established safety below age 25, in pregnancy or breastfeeding, or alongside medication.
The most frequent side effects
Cannabidiol produces none of the high linked to THC. The WHO expert committee, in its 2018 critical review, calls it generally well tolerated, with a good safety profile and no sign in humans of potential for abuse or dependence. A well-tolerated substance can still have effects.
A systematic review published in 2023 in Current Neuropharmacology examined the published clinical trials. It found mostly mild or moderate adverse effects, led by diarrhoea, drowsiness and sedation. The few serious cases mainly concerned patients who were also on anti-seizure drugs such as clobazam or valproate.
Epidyolex, the only CBD medicine authorised in the European Union, provides the firmest figures. At 10 to 25 mg/kg a day, its official product information (summary of product characteristics) lists these frequencies:
| Adverse effect | Frequency on Epidyolex |
|---|---|
| Drowsiness | 23% |
| Reduced appetite | 21% |
| Diarrhoea | 20% |
| Fever | 16% |
| Vomiting | 12% |
| Fatigue | 10% |
The patients were children and adults with severe epilepsy, many of them on several medicines. Their percentages cannot be applied to a healthy adult taking a few milligrams. They indicate the type of effects to expect and say nothing about their likelihood at low doses. For context on these trials, see our summary of what clinical studies on CBD really say.
The liver: what two trials in healthy volunteers found
The liver carries the most serious signal. Doctors follow it with a blood test of two enzymes, ALT and AST (the transaminases), whose levels rise when the liver is under stress.
A phase I trial in 2021 gave 1,500 mg of CBD a day to 16 healthy adults for about 3.5 weeks. ALT exceeded the upper limit of normal in seven of them (44%). Five (31%) passed 5 times that limit, the threshold that defines drug-induced liver injury. All the rises occurred between the second and the fourth week.
Researchers at the US FDA tested a dose nearer to consumer use in 2025. In their randomised placebo-controlled trial, 201 healthy adults took either 5 mg/kg of CBD a day (350 mg for 70 kg) or a placebo for 28 days. In the CBD group, eight participants (5.6%) had transaminases above 3 times the normal limit; none did on placebo. Seven met the withdrawal criteria for possible liver injury. Thyroid hormones and testosterone showed no change.
The authors ask for studies on long-term effects and on the lower doses many consumers take. How to read a blood test is explained in our article on CBD, liver enzymes and interactions.
Interactions with medicines
In practice, this point matters most. Many medicines are cleared by liver enzymes of the cytochrome P450 family, and the 2023 review reports that CBD strongly slows two of them, CYP3A4 and CYP2C19. A medicine that is cleared more slowly accumulates, and its side effects increase with it.
Measured examples appear in the Epidyolex product information:
- clobazam, an anti-seizure drug: the level of its active metabolite rises 3 to 4 times, with more drowsiness;
- valproate: raised transaminases become more frequent;
- everolimus, an immunosuppressant: exposure to the drug increases about 2.5 times.
Grapefruit juice works by a similar mechanism, explained step by step in our article on CBD interactions and the grapefruit effect. If you take a regular treatment, talk to your doctor or pharmacist before using any CBD product.
The dose changes everything: benchmarks in milligrams
Much of the confusion is a matter of scale. The fullest tolerability data concern medicinal doses of several hundred milligrams a day, while the EFSA benchmark sits several hundred times lower.
You can place a bottle on this scale with a little arithmetic. A 10% oil contains about 100 mg of CBD per millilitre. If the dropper gives 20 drops per millilitre, each drop holds roughly 5 mg and three drops hold 15 mg. That already exceeds the EFSA benchmark while staying far below the doses of the liver trials, and no data cover the range in between. The calculation for other strengths is in our article on EFSA's reference dose.
The CBD sold in shops is not a medicine and makes no therapeutic claim. Epidyolex is prescribed in rare forms of epilepsy, under medical follow-up with liver tests. Its trials are summarised in our feature on Epidyolex and epilepsy.
Who should avoid CBD, according to health agencies
EFSA published a provisional safe level on 9 February 2026: 0.0275 mg of CBD per kilo of body weight per day, or about 2 mg for a 70 kg adult. It builds in a safety factor for data gaps, in particular on the liver and on the endocrine, nervous and reproductive systems. The level applies only to CBD of at least 98% purity. EFSA concludes that safety cannot be established for people under 25, pregnant or breastfeeding women, and people on medication.
A second file concerns reproduction. On 21 March 2025 the French agency Anses proposed classifying CBD as presumed toxic to reproduction (category 1B), on the basis of animal studies that showed harm to fertility and development. The Belgian Federal Public Service for Health reports that the risk assessment committee of the European Chemicals Agency adopted an opinion supporting this classification. The European Commission has not yet decided. Our article on the classification proposed by Anses tracks the procedure, and the one on CBD during pregnancy and breastfeeding lists the precautions.
The last point is driving. In a joint alert of 19 June 2025, Anses and the French medicines agency ANSM note that even a compliant product contains a little THC, which can cause drowsiness or lethargy. They advise drivers and workers exposed to risks not to use CBD. Those traces can also turn a saliva test positive, as explained in our guide to saliva drug testing.
The most common risk: a mislabelled product
The same alert reports several hundred poisonings recorded since early 2024 in people who had taken products sold as CBD. In most cases the cause lay elsewhere: synthetic cannabinoids added without the buyer's knowledge (HHC, H4CBD, MDMB-PINACA) or THC above 0.3%. A study backed by Mildeca, the French interministerial drug agency, and cited in the release found that 8 products out of 10 had a CBD content different from their label.
The status of food containing CBD has changed too. On 20 May 2026 the French Ministry of Agriculture restated that foodstuffs containing CBD are not authorised for sale because their safety has not been demonstrated, and said checks would be extended. What can still be sold is detailed in our update on CBD regulation in France in 2026.
For a product you do not swallow, the one check you can make yourself is the batch certificate of analysis. Look for the same batch number as on the pouch, Δ⁹-THC at 0.30% or less, and no HHC or H4CBD where the lab tested for them. Our guide to reading a certificate of analysis covers each line, and the laboratory page describes the measurement methods.
Frequently asked questions
According to the WHO it is generally well tolerated and has no abuse potential. It still has effects: drowsiness, digestive upset, raised liver enzymes in some people, and changes in how certain medicines work. The risk increases with the dose.
Drowsiness, diarrhoea and reduced appetite. At 10 to 25 mg/kg a day of Epidyolex, they occur in 23%, 20% and 21% of patients respectively. In healthy adults they are usually mild.
It can raise transaminases: in the 2025 FDA trial, 5.6% of volunteers taking about 350 mg a day exceeded 3 times the normal limit. No data exist for lower doses. Ask a doctor if you have liver disease or take medication.
Only after you have talked to a doctor or pharmacist. CBD slows CYP3A4 and CYP2C19, enzymes that clear many medicines, anti-seizure drugs and immunosuppressants among them. For people on medication, EFSA considers its safety not established.
In the Epidyolex trials, drowsiness is the most frequent adverse effect. It is a reason to avoid driving after a dose and proves nothing about sleep. CBD does not get you high, as explained in our article does CBD get you high?.
Written by the Phytogrammes team
Each journal article draws on cited primary sources: ANSM, EFSA, EUR-Lex and peer-reviewed publications. Product profiles are distinguished from documented laboratory results. Our approach.
Article published on . CBD is not a medicine.