What are the side effects of cannabis-based medicines?

Most side effects in this field are mild, predictable and ease over the first few weeks. A small number are serious and a few are reasons to stop immediately. Knowing which is which is the point of this page.

Common effects, and what usually settles

The most frequently reported effects are dose-related and often diminish as the body adapts, which is one of the arguments for slow titration. They are also, for the most part, mild and familiar rather than exotic, which is why knowing what to expect takes much of the worry out of the first few weeks.

Effects that need prompt attention

A smaller group of effects should prompt you to contact your prescriber rather than wait for the next appointment.

• Marked anxiety, panic or paranoia — common enough to be worth naming, and a reason to review the dose promptly
• Confusion, hallucinations, or losing touch with reality — stop and seek medical advice the same day
• Fast or irregular heartbeat, chest pain — seek urgent medical attention
• Fainting, or repeated near-fainting on standing
• Persistent vomiting with abdominal pain, particularly if relieved by hot showers — this pattern needs assessment
• Worsening mood or thoughts of self-harm — contact your GP or NHS 111 the same day; in an emergency call 999

Psychosis is rare but is the effect that most shapes who gets assessed as suitable in the first place. It is why a personal or family history of psychotic illness will normally count against treatment, and why an honest account of your own and your family's mental health history at the assessment protects you rather than working against you. The screening that happens before a prescription is issued is largely there to keep this small risk small.

Interactions and other medicines

Cannabis-based medicines interact with a number of commonly prescribed drugs. This is not a side effect of the medicine alone but of the combination.

The combinations that most often matter involve sedatives, some blood thinners, and some antiepileptic medicines. Combining with anything else that causes drowsiness, including alcohol, can add to sedation and impairment, which is worth bearing in mind day to day. Your prescriber needs your full medicines list, including anything bought over the counter and any supplements, and your GP needs to know about the prescription for the same reason — see Should you tell your GP?

Reporting a side effect

The MHRA's Yellow Card scheme collects reports of suspected side effects from patients as well as clinicians. For unlicensed medicines, where formal trial data is limited, patient reports carry real weight.

Most cannabis-based products prescribed in the UK are unlicensed, which means they have not been through the full licensing process that builds up a large safety dataset before a medicine reaches the market. Reporting is one of the main ways that dataset is built afterwards, so a report about an effect you were not warned about genuinely adds to what is known — and it costs you nothing.

You can report directly without going through your clinic, you do not need to be certain the medicine caused the problem, and reporting does not affect your treatment. Tell your prescriber as well — they need to know regardless.

Why side effects happen, and why many ease

Most of the effects on this page follow from what the active compounds do in the body. Cannabinoids act on the endocannabinoid system, which is involved in regulating things like alertness, appetite, blood pressure control and mood, so effects turn up in those areas.

Most side effects are dose-related and are associated with THC in particular, which is the intoxicating component. That is the reasoning behind starting at a low amount and increasing slowly: it keeps effects within a manageable range and gives the body time to adapt. Many effects — drowsiness, dizziness, light-headedness — commonly settle over the first few weeks as that adaptation happens, which is one reason a specialist judges a trial over weeks rather than days. The two main compounds are described in THC and CBD explained, and the titration process in how a dose is decided and adjusted.

Adaptation is not guaranteed and not universal. Some effects, such as dry mouth, often persist, and a few people do not tolerate treatment at all. None of this predicts benefit, which varies widely by condition and between individuals.

Reducing the chance of side effects

You cannot remove the risk, but sensible habits keep it lower and make problems easier to manage. Most of these overlap with getting the treatment right in general.

• Go at the agreed pace. Increasing faster than your prescriber has set is the commonest self-inflicted cause of side effects.
• Be consistent with food and timing, particularly with swallowed preparations, so that effects are more predictable.
• Do not drive or operate machinery while affected; impairment and its duration differ by route, as covered in driving with a medical cannabis prescription.
• Manage dry mouth actively with fluids, sugar-free gum and dental care, because it raises the risk of tooth decay if ignored.
• Give your prescriber a complete medicines list, including anything bought over the counter and any supplements, so interactions can be anticipated.
• Keep a brief record of what you notice and when, which makes reviews more useful and helps separate a side effect from an unrelated problem.

Questions worth asking your specialist

Knowing in advance what to expect, and what to act on, makes side effects far less alarming. Useful questions include:

• Which side effects are most likely for me, and which usually settle?
• Which effects should make me contact you promptly rather than wait?
• Are there any that mean I should stop straight away?
• Do any of my existing medicines interact with this?
• How do we adjust the plan if a side effect does not settle?

A clinician who sets this out clearly, and who wants to hear about anything new, is managing the treatment properly rather than leaving you to work it out alone.

Patient guide · Written by the Flora House editorial team · Last reviewed: 7 October 2026