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.
The short answer
- Drowsiness, dry mouth and dizziness are common and often settle within weeks.
- Most side effects come from THC, and most are dose-related.
- Psychiatric effects are the serious ones — anxiety, paranoia, and rarely psychosis.
- Report side effects to the MHRA through the Yellow Card scheme.
- Tell your prescriber about anything new, even if it seems unrelated.
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.
| Effect | Typical course | What helps |
|---|---|---|
| Drowsiness or sedation | Often eases over two to four weeks | Timing doses differently; slower increases; not driving |
| Dry mouth | Usually persists to some degree | Fluids, sugar-free gum, dental care — dry mouth increases decay risk |
| Dizziness or light-headedness | Usually early and settles | Standing up slowly; reporting it if it persists |
| Appetite change | Variable, may be wanted or unwanted | Worth mentioning either way — it can be useful or a problem |
| Impaired concentration or memory | Dose-related | Dose review; avoiding safety-critical tasks |
| Nausea | Usually early | Taking with food where advised |
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.
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. 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.
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.
Frequently asked questions
What are the most common side effects?
Drowsiness, dry mouth, dizziness, appetite change, impaired concentration and nausea. Most are dose-related and many ease over the first two to four weeks as the body adapts.
Which side effects are serious?
Marked anxiety, panic or paranoia; confusion or hallucinations; fast or irregular heartbeat or chest pain; fainting; persistent vomiting with abdominal pain; and any worsening of mood or thoughts of self-harm. Seek medical advice promptly for these.
Do side effects go away?
Many do. Drowsiness and dizziness commonly settle within a few weeks. Dry mouth often persists and is worth managing actively because it increases the risk of tooth decay.
How do I report a side effect?
Through the MHRA Yellow Card scheme, which accepts reports directly from patients. You do not need to be certain the medicine caused it. Tell your prescriber as well.
Can it interact with my other medicines?
Yes. Interactions with sedatives, some blood thinners and some antiepileptic medicines are the ones that most often matter. Give your prescriber a complete medicines list, including anything bought over the counter.
Sources and further reading
Every factual statement on this page is drawn from the following public sources. Links open in a new tab and are not affiliated with Flora House.
- MHRA Yellow Card scheme: report a side effect
- British National Formulary: Cannabis extracts
- National Academies of Sciences, Engineering, and Medicine. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research (2017)
- NICE guideline NG144: Cannabis-based medicinal products
- NHS: Medical cannabis (and cannabis oils)
- MHRA: The supply, manufacture, importation and distribution of unlicensed cannabis-based products for medicinal use