CBD Safety and Drug Interactions: What to Know Before You Take It

CBD is often described as "completely safe" or "natural, so harmless". Neither statement is accurate. The overall safety picture is reasonably reassuring for healthy adults using modest amounts, but there are real side effects, real medicine interactions and groups of people who should avoid CBD. This guide covers what the research shows, labelled as human or preclinical, and what to do before you start.

The overall safety picture

In 2018 the World Health Organization's Expert Committee on Drug Dependence reviewed the evidence and reported that CBD was generally well tolerated, with no evidence of abuse or dependence potential human. Since then, more data have come in, much of it from the high-dose prescription trials in epilepsy. Those trials are the main source of information on side effects, and they used doses far above typical consumer amounts.

Common side effects

Side effects reported in human trials human include:

• tiredness or drowsiness;
• diarrhoea or loose stools, and stomach upset;
• reduced or changed appetite;
• dry mouth;
• light-headedness, especially when standing up quickly.

Most were mild to moderate and more common at higher doses. If you notice any of these, reduce the amount or stop.

The liver: the finding to take seriously

In the high-dose epilepsy trials, a proportion of participants developed raised liver enzymes (ALT and AST) on blood tests human. This happened more often in people also taking the anti-seizure medicine valproate. A later controlled study in healthy adults taking 1,500 mg a day for a few weeks also found raised liver enzymes in some participants human.

Those doses are much higher than consumer servings, and the changes generally settled after stopping. Even so, the findings are the main reason regulators have become more cautious about long-term daily use. Practical points:

• Avoid CBD if you have liver disease, unless your specialist agrees.
• Tell your GP you take CBD if you have routine blood tests.
• Stop and seek advice if you notice yellowing of the skin or eyes, dark urine, pale stools or unusual tiredness.
• Avoid combining CBD with heavy alcohol use.

How CBD interacts with medicines

Most medicine interactions with CBD come from the liver enzymes that process drugs, especially a family called cytochrome P450 (mainly CYP3A4 and CYP2C19). Researchers have observed that CBD can slow some of these enzymes human preclinical. If an enzyme slows down, another medicine that depends on it can build up in the blood, making side effects more likely. In some cases the reverse applies.

Medicines where interactions have been reported or are considered likely include:

A useful rule of thumb that pharmacists use: if a medicine label warns you to avoid grapefruit juice, ask before using CBD, because similar liver enzymes are involved.

Who should avoid CBD

• Pregnant or breastfeeding women. There is not enough safety information, and animal research has raised questions about effects on development preclinical.
• Children and teenagers, except under specialist medical care.
• People with liver disease, unless their specialist agrees.
• People taking medicines with known interactions (see above), unless their doctor or pharmacist agrees.
• People preparing for surgery: tell your surgical team, as with any supplement.

Driving, work and drug testing

CBD itself is not intoxicating, but it can make some people drowsy, especially at first. Don't drive or operate machinery until you know how it affects you. Drug tests look for THC rather than CBD. Products with trace THC could, rarely, contribute to a positive result, so choose products whose lab report shows THC as not detected. This matters for professional drivers and athletes in particular.

Product quality is a safety issue too

Independent testing of CBD products sold online in several countries has repeatedly found products that did not match their labels: some contained much less CBD than stated, some contained more, and some contained THC that was not declared. For safety, choose a product that:

• has a recent certificate of analysis from an independent lab, with a batch number matching your pack;
• shows THC as not detected;
• has been tested for contaminants such as heavy metals, pesticides and solvents;
• gives the amount of CBD per serving in milligrams;
• comes from a contactable seller with an Irish or EU address.

See CBD oil explained for how to read a lab report.

Starting sensibly

1. Check with your pharmacist if you take any medicine.
2. Start with a low amount and follow the label.
3. Increase slowly, if at all, and keep a simple diary.
4. Take it at the same time each day, ideally with food.
5. Stop if you notice side effects, and seek advice if you have any of the liver warning signs above.

Pets and CBD

CBD products for dogs and cats are widely sold. Small veterinary studies have looked at CBD in dogs animal, and some observed changes in liver enzyme tests similar to those seen in people. Never give your pet a human CBD product, as some contain ingredients such as xylitol that are toxic to dogs, and talk to your vet before starting, particularly if your pet takes any medication.

Older adults

Older people are more likely to take several medicines at once, which raises the chance of an interaction, and may be more sensitive to drowsiness and light-headedness, which can increase the risk of falls. If you are buying CBD for an older relative, involve their GP or pharmacist and start with the lowest amount on the label.

Long-term use: what we don't know yet

Most human safety data come from studies lasting weeks or a few months. There is far less information about taking CBD every day for years. Until better long-term data exist, a sensible approach is to use the lowest amount that seems worthwhile, take breaks, review whether it is actually helping you, and keep your GP informed. Researchers are particularly interested in long-term effects on the liver, hormones and the developing brain, which is one reason children, teenagers and pregnant women are advised to avoid CBD.

Sources

1. World Health Organization Expert Committee on Drug Dependence. Cannabidiol (CBD) critical review report, 2018.
2. Devinsky O et al. Trial of cannabidiol for drug-resistant seizures in the Dravet syndrome. New England Journal of Medicine, 2017.
3. Watkins PB et al. Cannabidiol and abnormal liver chemistries in healthy adults: results of a phase I clinical trial. Clinical Pharmacology and Therapeutics, 2021.
4. Brown JD, Winterstein AG. Potential adverse drug events and drug-drug interactions with medical and consumer cannabidiol (CBD) use. Journal of Clinical Medicine, 2019.
5. Bonn-Miller MO et al. Labeling accuracy of cannabidiol extracts sold online. JAMA, 2017.