Medical Cannabis Side Effects and Interactions

The main safety question with medical cannabis is not whether side effects exist, but how likely they are to affect day-to-day functioning and other medicines.

Key takeaways

• Common side effects include drowsiness, dizziness, diarrhoea, feeling sick, weakness, appetite change, feeling high, and mood change.
• More concerning effects can include hallucinations and suicidal thoughts.
• THC is more likely than CBD alone to cause intoxication, impaired thinking, and psychosis risk.
• CBD and THC can interact with other medicines, and CBD can affect liver function.
• Do not drive if you feel impaired, even if the medicine is prescribed.
• Pregnancy and breastfeeding need specialist advice; evidence is limited and some products are not suitable.

Evidence base

The NHS medical cannabis page lists common side effects and says CBD and THC can affect how other medicines work. It also says doctors need to monitor liver function because CBD can affect how the liver works.

NICE adds the prescribing safety context. Clinicians should consider:

• dependence, diversion, and misuse, especially with THC
• mental health and medical history
• liver, kidney, and cardiovascular disease
• interactions with central nervous system depressants, other centrally active drugs, antiepileptics, and hormonal contraceptives
• pregnancy and breastfeeding, where evidence is limited and breastfeeding is a contraindication for some products

NHS England notes that the evidence base is still developing and that long-term harms have not been well studied. That matters because short-term tolerance is not the same as long-term safety.

For driving, GOV.UK guidance says it is illegal to drive if you are unfit because of drugs or medicine, and the key practical point is to avoid driving if you feel impaired.

What patients should know

The risk of side effects depends on the product, dose, and THC content. A CBD-only medicine is different from a product containing THC, and many real-world products contain more than the label suggests.

Watch for:

• sleepiness or sedation
• dizziness or unsteadiness
• slowed thinking, poor concentration, or memory problems
• mood changes, anxiety, or low mood
• feeling intoxicated or "high"
• nausea, diarrhoea, or reduced appetite

These effects are more likely when starting treatment, increasing dose, or combining the product with other medicines that also cause sedation. They can be especially important if you already have depression, anxiety, psychosis, seizure disorders, liver disease, or cardiovascular disease.

If you take other medicines, do not guess about interactions. Ask the prescriber or a pharmacist. This is particularly important with medicines that affect the brain, medicines for epilepsy, some contraceptives, and treatments that are processed by the liver.

If you are pregnant, trying to conceive, or breastfeeding, do not assume a cannabis-based product is harmless just because it is sold as CBD. Get a clinical review first.

When to speak to a clinician

Speak to a clinician if:

• side effects start after you begin or change dose
• you feel too sleepy, muddled, anxious, or dizzy to function normally
• you have hallucinations, severe confusion, or suicidal thoughts
• you are unsure whether you can drive
• you are pregnant, breastfeeding, or planning pregnancy
• you take regular medication and want interaction advice
• you have a history of psychosis or significant mood disorder

If symptoms are severe or you feel unsafe, seek urgent help rather than waiting for the next routine review.

Source trail

• NHS medical cannabis page: common side effects, interactions, and liver monitoring.
• NICE NG144: interaction checks, mental health and medical history, pregnancy/breastfeeding, and driving advice.
• NHS England CBPMs: limited evidence base and long-term harms not well studied.
• GOV.UK drug-driving guidance: do not drive if you are impaired by medicine, even if it is prescribed.