Medical cannabis can cause side effects. That does not mean every patient will have a bad experience, and it does not mean prescribed medical cannabis is the same thing as unregulated cannabis bought online. It means the safety side needs to be treated properly: side effects should be discussed with the prescribing medical team, serious or frightening symptoms need urgent help, and suspected reactions can be reported through the MHRA Yellow Card scheme.
The useful version is this: if a prescribed cannabis-based medicine is making you feel unwell, more anxious, unusually low, impaired, confused, or worried about another medicine you take, do not try to solve that alone. Tell the medical team responsible for the prescription. If you feel at immediate risk of harm, use urgent NHS help.
This guide is general UK patient information, not medical advice. It does not tell you whether medical cannabis is suitable for you, what to take, how to take it, or how to change a prescription.
What side effects can medical cannabis cause?
The NHS lists several possible side effects from medical cannabis. They can include:
- decreased appetite
- diarrhoea
- feeling sick
- weakness
- a behavioural or mood change
- dizziness
- feeling very tired
- feeling high
- hallucinations
- suicidal thoughts
That list matters because some of these are easy to dismiss as a bad day, stress, tiredness, or part of the original condition. Sometimes that may be true. Sometimes it may not. If a symptom starts after a new prescription, gets worse, feels unusual for you, or worries you, it belongs in a conversation with the prescribing medical team.
Side effects can also sit on a spectrum. Feeling slightly more tired is not the same as feeling unsafe, severely confused, or unable to function. A mild stomach upset is not the same as a severe or persistent reaction. The article cannot sort those cases for an individual patient. The prescriber has the medical history, medicine list, and clinical responsibility needed to judge what is happening.
Mental health symptoms need particular care
The NHS includes mood or behaviour change, hallucinations, and suicidal thoughts in its side-effect list. These are not small print.
If medical cannabis appears to make your mental health worse, or if you notice thoughts or behaviour that feel frightening, out of character, or unsafe, contact the prescribing medical team promptly. If you feel like you may harm yourself, have already harmed yourself, or feel that your life is in danger, call 999 or go to A&E. The NHS also lists NHS 111 and crisis support routes for people experiencing suicidal thoughts.
This is where MCPH needs to be plain rather than dramatic. Medical cannabis may be helpful for some suitable patients, but it is still a medicine that can affect mood, perception, alertness, and judgement. Patient safety beats optimism every time.
Interactions with other medicines
The NHS says CBD and THC can affect how other medicines work. That is one reason medical cannabis should not be treated as separate from the rest of a patient's care.
Interactions can matter when someone is taking prescribed medicines, over-the-counter medicines, supplements, or has a condition where monitoring is already important. The NHS also says CBD can affect how the liver works, meaning doctors may need to monitor patients regularly.
The practical point is not that patients need to become medicine experts. It is that the prescribing medical team needs an accurate picture of the medicines and health factors involved. If another medicine has changed, a new symptom has appeared, or a side effect feels linked to a medicine combination, that is information for the clinician, not something to guess around.
Feeling impaired and driving risk
Some side effects are directly relevant to driving and safety-critical tasks. Dizziness, tiredness, feeling high, mood change, confusion, or slowed judgement can all matter in the real world.
GOV.UK says it is illegal to drive if you are unfit because of legal or illegal drugs. Prescription medicines are legal drugs, but a prescription is not a free pass to drive while impaired. If a medicine affects alertness, judgement, reaction time, or coordination, the safe and legal boundary needs proper medical advice.
This article cannot give driving clearance. If driving is relevant to your work, caring responsibilities, or daily life, discuss it with the prescribing medical team before relying on assumptions.
Why unlicensed medicines make monitoring important
A lot of cannabis-based medicinal products in the UK are unlicensed. That does not automatically mean they are wrong for a patient. It does mean the governance bar is higher.
NHS England, GMC, and CQC guidance all point to the same basic safety idea: prescribing decisions need proper clinical responsibility, enough patient information, and attention to benefits, risks, evidence, other medicines, and monitoring. CQC also notes that unlicensed medicines can carry higher risk because they may not have been assessed for safety, quality, and efficacy in the same way as licensed medicines.
For patients, the takeaway is simple. Side effects are not a personal failure, and they are not something to hide because you are worried the prescription will be questioned. They are part of the safety picture the prescriber needs to see.
Reporting side effects
The NHS says side effects from medical cannabis should be reported to the medical team. NICE and GMC also point adverse-event reporting to the MHRA Yellow Card scheme.
Yellow Card is the UK system for reporting suspected side effects and safety issues with medicines and medical devices. The MHRA says patients and members of the public can report suspected side effects. You do not have to prove that the medicine definitely caused the reaction before reporting a suspicion.
Yellow Card is not a replacement for medical help. If you are worried about your health, MHRA signposts people to speak to a doctor or call NHS 111. If symptoms are urgent or dangerous, use urgent NHS routes rather than waiting for an online report to be reviewed.
What not to do
Do not adjust, pause, restart, combine, or add medicines without prescriber guidance. That includes prescribed medicines, over-the-counter products, and cannabis products bought outside the prescription pathway.
Do not treat a side effect as proof that medical cannabis is automatically unsuitable for every patient. It may mean the prescription needs review, the symptom has another cause, an interaction needs checking, or the risks now outweigh the benefits for that person. Those are clinical decisions.
Do not assume natural means harmless. Cannabis-based medicines can have real effects on mood, perception, alertness, other medicines, and liver monitoring.
Do not drive if you feel impaired. Prescription status does not remove the legal risk if a drug affects your ability to drive.
Where this fits with MCPH
If you are trying to understand the wider UK prescription pathway, MCPH has a separate guide to the medical cannabis prescription process. If your question is specifically about cannabidiol, the existing MCPH article on CBD side effects may be the better next read.
For the site's general safety boundary, read the MCPH medical disclaimer. For cannabinoid background, the MCPH THC guide gives educational context, but it is not a substitute for prescriber advice.
More from the MCPH Patient Guide
Use the MCPH Patient Guide to follow the UK medical cannabis pathway in order, from eligibility and records through to safety, side effects and review questions.
Source trail
- NHS: Medical cannabis (and cannabis oils): https://www.nhs.uk/medicines/medical-cannabis/
- NICE NG144: Cannabis-based medicinal products: https://www.nice.org.uk/guidance/ng144
- NHS England: Cannabis-based products for medicinal use: https://www.england.nhs.uk/long-read/cannabis-based-products-for-medicinal-use-cbpms/
- GMC: Information for doctors on cannabis-based products for medicinal use: https://www.gmc-uk.org/professional-standards/learning-materials/information-for-doctors-on-cannabis-based-products-for-medicinal-use
- CQC: Cannabis-based medicinal products, what CQC expects from providers: https://www.cqc.org.uk/guidance-providers/healthcare/cannabis-based-medicinal-products-what-cqc-expects-providers
- MHRA Yellow Card: https://yellowcard.mhra.gov.uk/
- GOV.UK: Yellow Card scheme guidance: https://www.gov.uk/guidance/the-yellow-card-scheme-guidance-for-healthcare-professionals-patients-and-the-public
- GOV.UK: Drugs and driving, the law: https://www.gov.uk/drug-driving-law
- NHS: Help for suicidal thoughts: https://www.nhs.uk/mental-health/feelings-symptoms-behaviours/behaviours/help-for-suicidal-thoughts/
