Last updated: 28 June 2026.
CBD is one of the cannabinoids patients hear about most, partly because it appears in both prescription medicines and ordinary high-street products.
That can make the subject more confusing than it needs to be. CBD is not the same thing as THC, and it does not produce the same intoxicating effect. But it is still an active substance. It can cause side effects, it can interact with medicines, and the context matters.
The useful version is this: CBD is not something to be scared of, but it should not be treated as magic wellness oil either. If you are already prescribed medical cannabis, or you are speaking to a clinic about it, the sensible step is to tell the clinician what CBD products, medicines and supplements you already use.
CBD in prescription medical cannabis context
In UK medical cannabis conversations, CBD can mean a few different things.
It can mean a licensed CBD medicine, such as cannabidiol used for specific severe epilepsies. It can also mean CBD contained within a cannabis-based medicinal product. Separately, it can mean a non-prescription CBD product sold as a food supplement.
Those are not all the same thing.
Prescription medicines are assessed and prescribed in a medical context. Non-prescription CBD products are not a shortcut to the same standard. The NHS warns that products sold online or in shops may not have reliable quality, content or health benefits.
That does not mean every CBD product is dangerous. It means patients should be clear about what they are taking and avoid assuming that a product is clinically useful just because it says CBD on the label.
Common CBD side effects
CBD side effects are usually discussed in a calm, practical way. They can include things like:
- sleepiness or tiredness
- diarrhoea or stomach upset
- appetite changes
- feeling sick, weak, tired, or generally unwell
- changes in liver blood tests in some prescription contexts
Not every patient gets side effects. Some people tolerate CBD well. Others find that even a product that sounds gentle on paper does not agree with them.
The point is not to panic. The point is to notice what changes, tell the prescriber or pharmacist, and avoid quietly adding CBD on top of other medicines without mentioning it.
CBD and other medicines
CBD and THC can affect how other medicines work. That is one of the reasons clinics ask about current medicines, supplements, and any cannabis or CBD products a patient already uses.
This should not be framed as frightening. Interaction checks are a normal part of prescribing. A specialist clinician or pharmacist can look at what matters for your situation.
The useful action for patients is simple:
- tell the clinic about prescribed medicines
- mention over-the-counter medicines
- mention supplements
- mention CBD oils, gummies, capsules, vapes, or other cannabis products
- do not stop prescribed medicines without medical advice
For most patients, the clinic is not expecting them to become an interaction expert. They need an accurate list so the prescribing team can make a sensible decision.
Prescription CBD and liver monitoring
The NHS notes that CBD can affect how the liver works, which is why doctors may need to monitor patients in some prescription contexts.
That does not mean every person who has ever used CBD needs to worry. It means liver monitoring can be part of safe prescribing, especially where higher-dose prescription CBD or complex medicines are involved.
If you already have liver disease, abnormal liver blood tests, or you take medicines that need monitoring, this is worth telling the clinic early.
CBD bought online or from shops
A lot of CBD content online blurs the line between medical information and product marketing. MCPH should not do that.
In the UK, CBD products sold as food supplements sit in a different category from prescribed cannabis-based medicines. The MHRA says CBD products marketed for medical purposes are medicines, and authorised medicines have to meet safety, quality and efficacy standards.
The Food Standards Agency also treats CBD as a novel food issue for non-medical products in England and Wales.
For patients, the practical point is this: if you are using non-prescription CBD, tell the clinic. Do not assume it is irrelevant because it came from a shop rather than a pharmacy.
When to ask for help
Speak to the prescribing clinic, pharmacist, GP surgery, or another appropriate healthcare professional if:
- side effects are difficult or worrying
- you feel unusually sleepy, unwell, or mentally different after starting a CBD-containing product
- you are taking several medicines and are not sure what to disclose
- you have liver disease or have been told your liver blood tests are abnormal
- you are pregnant, breastfeeding, trying to conceive, or using CBD in a child or young person
If symptoms are urgent or severe, use NHS 111, 999, or local emergency services depending on the situation.
Side effects from medicines can also be reported through the MHRA Yellow Card scheme. That is separate from getting medical help if something feels urgent or unsafe.
The MCPH view
CBD can be part of useful patient education. It should not be sold to patients as a harmless wellness shortcut.
In medical cannabis care, the better frame is preparation: know what you are taking, tell the clinician, and let the prescribing team decide what matters.
Useful next reads
- Medical cannabis side effects
- Medical cannabis and medication interactions
- CBD vs THC in prescription context
- Patient Guide
