The practical answer is this: THC can be part of prescribed medical cannabis, but it is also the compound most closely linked with feeling high and being impaired. That matters because impairment is not just uncomfortable. It can affect safety, medicine reviews, work, caring responsibilities, and driving.
This guide is general UK patient information. It is not medical advice, legal advice, dosing advice, or a way to decide whether you are clear to drive. If side effects are happening, the right place to take them is your prescriber, medical team, pharmacist, or another qualified healthcare professional.
For wider cannabinoid context, MCPH has a short page on THC and a separate page on CBD. This article is about the safety question: what side effects and impairment can mean in a prescribed medical cannabis context.
What THC is doing in this conversation
THC is the cannabis compound most associated with psychoactive effects. NHS medical cannabis guidance describes THC as the chemical in cannabis that makes people feel high, while CBD does not have that same effect when a medicine contains no THC.
That does not mean every patient will feel the same thing from a prescribed product containing THC. It does mean THC needs proper clinical boundaries. NICE tells prescribers to consider dependence risk, mental health and medical history, interactions with other medicines, and how cannabis-based medicinal products may affect ability to drive.
In normal language: THC is not just another ingredient name. It is a safety variable your prescriber needs to understand in the context of your health, other medicines, and daily responsibilities.
Side effects named by the NHS
NHS medical cannabis guidance says possible side effects depend on the type of medical cannabis used. The listed effects include:
- decreased appetite
- diarrhoea
- feeling sick
- weakness
- behavioural or mood change
- dizziness
- feeling very tired
- feeling high
- hallucinations
- suicidal thoughts
That list is worth taking seriously without turning it into panic. A mild side effect and a frightening side effect are not the same thing, but both are information for the medical team. NHS guidance says side effects from medical cannabis should be reported to the medical team and can also be reported through the MHRA Yellow Card Scheme.
MCPH already has a related guide on CBD side effects, but THC raises some different questions because impairment, psychoactive effects, dependence risk, and driving boundaries can become part of the review.
Impairment is the bit to be honest about
Impairment does not have to look dramatic. It may show up as dizziness, tiredness, feeling high, confusion, mood change, or not feeling able to judge things normally. Hallucinations or suicidal thoughts are more serious warning signs and need prompt help.
If you are worried about your health, the MHRA Yellow Card site points people to a doctor, pharmacist, or NHS 111. If a mental health crisis means life is at risk, or you do not feel able to keep yourself or someone else safe, NHS urgent mental health guidance says to call 999 or go to A&E.
Do not try to tidy this up on your own by guessing what caused it, changing how you use your medicine, or adding non-prescribed cannabis or online products. That is exactly the kind of situation where the prescriber needs a clear account of what happened, when it happened, what else you were taking, and whether anything felt unsafe.
Driving: the official boundary
MCPH cannot give driving clearance. The official starting point is GOV.UK drug-driving law.
GOV.UK says it is illegal to drive if you are unfit because of legal or illegal drugs. Legal drugs include prescription and over-the-counter medicines. GOV.UK also says that in England, Scotland, and Wales, prescription medicines can still create a driving offence if they impair driving. Northern Ireland is not covered by the specified-limits part of that GOV.UK page, but the page still notes that a person can be arrested if unfit to drive.
That is the boundary this article can safely give. A prescription is not a blanket answer to impairment. If there is any doubt about driving while prescribed a medicine, GOV.UK points patients towards a doctor, pharmacist, or healthcare professional.
NICE also expects prescribers to discuss how a cannabis-based medicinal product may affect ability to drive before prescribing. That conversation belongs with the prescribing team, not a blog post.
Interactions and higher-risk situations
NHS guidance says CBD and THC can affect how other medicines work, and that possible interactions should be discussed with a specialist. NICE adds more detail for prescribers, including interactions with central nervous system depressants, centrally active drugs, antiepileptics, and hormonal contraceptives.
NICE also says prescribers should think about mental health and medical history, including liver impairment, renal impairment, cardiovascular disease, pregnancy and breastfeeding, and history of substance misuse. For babies, children, and young people, NICE highlights possible effects on psychological, emotional, cognitive, structural, and functional brain development, and possible sedation.
The patient version is simple enough: side effects are not separate from the rest of your health. They can be tied to other medicines, past history, current symptoms, and the reason the medicine was prescribed in the first place.
When to contact the prescriber or medical team
Contact the prescribing team, medical team, pharmacist, or another qualified healthcare professional if:
- side effects feel new, strong, worrying, or hard to explain
- you feel high, dizzy, unusually tired, confused, or not yourself
- mood, behaviour, anxiety, hallucinations, or suicidal thoughts appear or worsen
- you are worried about interactions with another medicine
- you are unsure how the medicine affects work, caring, driving, or other safety-critical tasks
- someone else notices changes in your behaviour or alertness
This is not about being difficult. It is part of safe prescribing. NICE says efficacy and safety should be monitored and evaluated, and that adverse effects should be recorded. GMC guidance also points doctors towards Yellow Card reporting for adverse reactions.
If you need the broader treatment-pathway context, read MCPH's guide to how the medical cannabis prescription process works in the UK. For the general medical boundary on this site, see the MCPH medical disclaimer.
Useful questions for a review
A good review does not need theatre. It needs clear facts.
Useful questions include:
- Which side effects should I report straight away?
- Could this be interacting with any of my other medicines?
- What does my patient information leaflet say about this effect?
- Who do I contact if this happens again?
- How are side effects and outcomes being recorded?
- What official guidance applies if this could affect driving or safety-critical work?
- Are there any mental health, liver, kidney, cardiovascular, pregnancy, breastfeeding, or dependence risks that change the review?
These questions keep the conversation where it belongs: with the clinician or prescriber responsible for the medicine.
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 – https://www.nhs.uk/medicines/medical-cannabis/
- NICE NG144: Cannabis-based medicinal products recommendations – https://www.nice.org.uk/guidance/ng144/chapter/Recommendations
- GOV.UK: Drugs and driving: the law – https://www.gov.uk/drug-driving-law
- 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
- NHS England: Cannabis-based products for medicinal use – https://www.england.nhs.uk/long-read/cannabis-based-products-for-medicinal-use-cbpms/
- MHRA Yellow Card – https://yellowcard.mhra.gov.uk/
- NHS: Where to get urgent help for mental health – https://www.nhs.uk/nhs-services/mental-health-services/where-to-get-urgent-help-for-mental-health/
