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CBD is generally well tolerated, but it is not free of side effects. The ones reported most often in clinical trials are tiredness, diarrhea, changes in appetite and, at high doses, raised liver enzymes. Most are mild and dose-related. The more important issue for many people is that CBD can interact with prescription medicines, because it is processed by the same liver enzymes as many common drugs.
This guide covers what the research actually shows, how dose changes the picture, which medicines deserve a conversation with your doctor, and who should avoid CBD altogether. It is general information, not medical advice.
Common side effects of CBD oil
The best safety data on CBD comes from the clinical trials behind Epidiolex, a prescription CBD medicine approved for certain rare forms of epilepsy. Those trials used very high doses, 10 to 20 mg per kilogram of body weight per day, which for a 70 kg adult means 700 to 1,400 mg daily. That is many times more than a typical wellness serving of 10 to 50 mg, so treat the percentages below as an upper bound rather than what most people will experience.
| Side effect | CBD 10 mg/kg/day | CBD 20 mg/kg/day | Placebo |
|---|---|---|---|
| Sleepiness (somnolence) | 23% | 25% | 8% |
| Decreased appetite | 16% | 22% | 5% |
| Diarrhea | 9% | 20% | 9% |
| Raised liver enzymes | 8% | 16% | 3% |
| Fatigue or weakness | 11% | 12% | 4% |
| Rash | 7% | 13% | 3% |
| Insomnia or disturbed sleep | 11% | 5% | 4% |
Source: Epidiolex US prescribing information, Lennox-Gastaut and Dravet syndrome trials.
Two patterns stand out. First, most side effects rise with the dose, which is a good reason to start low. Second, many trial participants were also taking other anti-seizure drugs, and some side effects, especially sleepiness and liver changes, were far more common in that group.
Tiredness and drowsiness
Feeling sleepy is the most frequently reported effect. It tends to appear early, when someone starts or increases their serving, and often settles after the first week or two. Alcohol, sleeping tablets, antihistamines and other sedating medicines can add to it. Until you know how CBD affects you, avoid driving or operating machinery after a larger serving.
Digestive effects: diarrhea and appetite changes
Loose stools and a reduced appetite are the next most common complaints, and both are clearly dose-related. With oils, the carrier oil can also play a part, as large amounts of any oil can upset a sensitive stomach. Taking CBD with food, splitting the daily amount into two servings, or reducing the serving usually helps.
Dry mouth, lightheadedness and mood changes
Some people notice a dry mouth or feel briefly lightheaded, particularly after a larger serving. Occasional reports of irritability or restlessness also exist. These effects are less consistent in the research and tend to fade when the serving is reduced.
Is CBD hard on the liver?
This is the side effect that matters most, because it is not something you can feel. In the Epidiolex trials, a significant rise in the liver enzyme ALT (more than three times the upper limit of normal) occurred in about 13% of patients, compared with 1% on placebo. The risk was strongly dose-related and was concentrated in people also taking the epilepsy drugs valproate and clobazam: around 30% with both, versus about 3% with neither. In nearly all cases the levels returned to normal when the dose was lowered or stopped.
At everyday wellness amounts, liver problems appear to be uncommon, but long-term data at those doses is still limited. In October 2023 the UK Food Standards Agency advised healthy adults to keep their intake to no more than 10 mg of CBD a day, citing a rising risk of liver and thyroid effects with higher intake over long periods. If you have a liver condition, drink heavily or take medicines that affect the liver, speak to your doctor before using CBD.
CBD drug interactions
CBD is broken down by the liver’s cytochrome P450 enzymes, mainly CYP3A4 and CYP2C19, and it can slow down how those enzymes process other drugs. When that happens, the level of the other medicine in your blood can rise. This is the same mechanism behind the well-known warning about grapefruit juice.
Medicines worth raising with your doctor or pharmacist before you start CBD include:
- Anti-seizure medicines, especially clobazam (CBD raised its active metabolite around threefold in trials) and valproate (higher risk of liver enzyme changes).
- Blood thinners such as warfarin, where case reports describe raised INR levels after starting CBD.
- Sedatives and sleep medicines, including benzodiazepines, as the drowsiness can add up.
- Some immunosuppressants, such as tacrolimus and everolimus, which have narrow safety margins.
- Any medicine with a “grapefruit warning” on the leaflet, which is a quick sign it is processed by CYP3A4.
This list is not complete. If you take any regular prescription medicine, check with a pharmacist before adding CBD, and do not stop or change a prescribed medicine on your own.
Who should avoid CBD or check with a doctor first
- Pregnant or breastfeeding women, and people trying to conceive. There is no adequate human safety data, and animal studies have raised developmental concerns.
- Children, unless CBD is prescribed and monitored by a doctor.
- Anyone taking prescription medicines, for the interaction reasons above.
- People with liver disease or abnormal liver test results.
- Anyone subject to workplace or sports drug testing, if they are considering a full spectrum product. CBD itself is not what tests look for, but trace THC can be. Our guide to whether CBD shows up on a drug test explains the risk by product type.
Does product quality affect side effects?
Yes, and it is one of the most overlooked factors. A 2017 study in JAMA tested 84 CBD products sold online and found that nearly 70% were mislabeled, with some containing undeclared THC. Contaminants such as pesticides, heavy metals and residual solvents can also cause problems that have nothing to do with CBD itself.
The practical defence is a batch-specific certificate of analysis that shows the cannabinoid content and screens for contaminants. Our guide to reading a CBD lab report shows what to check.
How to lower the risk of side effects
- Start low. Begin with a small serving, such as a few milligrams, and stay there for a week before adjusting. Our CBD oil strength guide shows how to work out milligrams per drop.
- Increase slowly. Raise the amount gradually and only if you need to, keeping an eye on how you feel.
- Take it with food if you notice stomach upset.
- Avoid alcohol and sedatives around larger servings.
- Check your medicines with a pharmacist before you start.
- Buy tested products with a batch-matched certificate of analysis.
When to stop taking CBD and seek advice
Stop and contact a doctor if you notice yellowing of the skin or eyes, dark urine, pain in the upper right side of your abdomen, unusual tiredness that does not settle, a rash, or any reaction that worries you. These can be signs of a liver problem or an allergic reaction and should be checked.
Is CBD oil the same as hemp seed oil?
No. Hemp seed oil is pressed from the seeds and contains little or no CBD, so its side effect profile is that of a food oil. CBD oil is an extract of the hemp plant’s flowers and leaves diluted into a carrier oil. If you are not sure which one you have, our guide to hemp seed oil vs CBD oil explains how to tell them apart from the label.
Frequently asked questions
What are the most common side effects of CBD oil?
Tiredness, diarrhea, reduced appetite and, at high doses, raised liver enzymes. Most are mild and become more likely as the dose increases.
Can CBD make you feel worse before you feel better?
Some people feel drowsy or have mild stomach upset in the first days, especially if they start with a large serving. These effects usually ease, and starting with a lower serving often avoids them.
Can CBD damage your liver?
At the very high doses used in epilepsy trials, raised liver enzymes were common, particularly alongside certain anti-seizure drugs. At typical wellness amounts the risk appears low, but long-term evidence is limited, so people with liver conditions should speak to a doctor first.
Does CBD interact with medications?
Yes, it can. CBD affects the liver enzymes that process many drugs, including some anti-seizure medicines, blood thinners and sedatives. Check with a pharmacist or doctor before combining CBD with any regular medication.
How long do CBD side effects last?
Mild effects such as drowsiness typically last a few hours after a serving and often lessen over the first one to two weeks of use. Our guide to how long CBD stays in your system covers the timing in more detail.
Can you take too much CBD?
There is no known fatal dose, but side effects become more likely as the amount rises. The UK Food Standards Agency advises healthy adults to take no more than 10 mg a day.
Does CBD get you high?
No. CBD is not intoxicating. Full spectrum products contain trace THC within legal limits, which is far below the amount needed to cause a high.
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