If you have started prescribed medical cannabis, a side-effect diary can make follow-up safer and less vague. It is not there to prove that the treatment is working. It is a simple record you can share with your prescriber, clinic or pharmacist if something feels unclear, uncomfortable or worrying.
Medical cannabis can cause side effects. NHS patient guidance lists possible effects including feeling sick, dizziness, feeling very tired, mood or behaviour changes, hallucinations and suicidal thoughts. It also says side effects should be reported to your medical team, and that CBD and THC can affect how other medicines work.
That is the point of a diary: it helps you describe what actually happened, without guessing what caused it or changing your treatment plan on your own.
What to record each time
Keep the diary short enough that you will actually use it. A few clear notes are usually more useful than a long entry written days later.
Useful things to record include:
- the date and time you noticed the side effect;
- what you felt, using plain words such as sleepy, dizzy, sick, anxious, confused, unusually low in mood, dry mouth or headache;
- how strong it felt, for example mild, moderate or hard to manage;
- how long it lasted;
- what prescribed product and form you had used, copied from the pharmacy label if helpful;
- whether you had used any other medicines, supplements, alcohol, caffeine or non-prescribed cannabis around the same time;
- what you were doing, such as resting, eating, working, travelling or trying to sleep;
- whether the effect settled, got worse, came back, or affected normal activities.
You do not need to diagnose the side effect. You are just making it easier for the clinical team to understand the pattern.
What not to do with the diary
A diary is not a dosing tool. Do not use it to decide on your own to take more, take less, stop, switch product, change route, or change timing. Those decisions belong with the prescriber.
It is also not a legal or driving-clearance record. If you feel drowsy, dizzy, confused, impaired or not fully alert, treat that as a safety issue and speak to your clinic or pharmacist. Do not use the diary as proof that driving or carrying on as normal is appropriate.
The diary is most useful when it keeps the facts separate from the interpretation. Write down what happened, then ask the clinic or pharmacy what it means for your treatment.
When to contact your clinic or pharmacist
Contact your clinic, pharmacist or usual care route if a side effect is worrying, persistent, hard to manage, or different from what you were told to expect. You should also contact them if another medicine has been started, stopped or changed, because interaction questions are a normal part of prescribing and pharmacy review.
It can help to say:
- “I have been keeping a side-effect diary. Would you like me to send it before my review?”
- “This side effect keeps happening. Should I report it now or wait until my next appointment?”
- “Could this be connected to another medicine or supplement I take?”
- “Are there any symptoms you want me to treat as urgent?”
- “What details do you need before deciding whether the plan is still suitable?”
If the clinic has given you specific urgent instructions, follow those instructions.
When to use NHS urgent help
Use NHS 111 if you think you need medical help now and you are not sure what to do, or if you are worried about your health and cannot get timely advice from your usual care route. NHS 111 can direct you to the right service.
Call 999 or go to A&E for a life-threatening emergency. NHS guidance says to call 999 for suspected anaphylaxis, including sudden swelling of the lips, mouth, throat or tongue, struggling to breathe, throat tightness, severe confusion, severe drowsiness, sudden dizziness with collapse, or someone fainting and not waking up.
Take suicidal thoughts seriously. NHS patient guidance lists suicidal thoughts as a possible side effect of medical cannabis. If you feel at immediate risk of harming yourself, call 999 or go straight to A&E. If you need urgent mental health help but your life is not in immediate danger, NHS 111 can help you find support.
You can still record what happened afterwards, but do not wait to finish the diary before getting help.
Reporting side effects
NHS medical cannabis guidance says side effects should be reported to your medical team. NICE and GMC guidance also point adverse reaction reporting toward the MHRA Yellow Card scheme, and Yellow Card accepts reports from patients as well as professionals.
That means there are two different actions:
- tell your clinic, pharmacist or medical team so they can advise on your care;
- report suspected side effects through the MHRA Yellow Card scheme where appropriate.
Yellow Card does not give personal medical advice. Its own site tells people who are worried about their health to speak to a doctor, pharmacist or NHS 111.
What to bring to a treatment review
Before a review, bring the diary in the simplest form you have. That might be notes on your phone, a paper list, a spreadsheet, or a short summary.
The most useful summary usually covers:
- the side effects that happened more than once;
- anything severe, frightening or unusual;
- whether side effects affected work, sleep, driving, caring responsibilities or daily tasks;
- whether symptoms changed after a prescription, medicine, health or routine change;
- any questions you want answered before the next prescription.
If you already keep private stock, usage or side-effect notes, bring the parts that are relevant. The aim is not perfect tracking. It is safer, clearer communication with the people responsible for your care.
What this guide cannot decide for you
This guide cannot tell you whether a side effect is expected, whether medical cannabis is suitable for you, or whether your treatment plan should continue. It also cannot replace the patient information leaflet, pharmacy advice, clinic instructions or urgent NHS help.
Everyone responds differently. What feels mild for one person may be hard to tolerate for another. Use the diary to describe your experience, then let your prescriber, clinic or pharmacist interpret it with you.
Read next
- Medical cannabis side effects
- What to do if medical cannabis makes symptoms worse
- What to track after starting prescribed medical cannabis
- Patient guide: getting started with medical cannabis in the UK
Sources
- NHS: Medical cannabis
- NHS: When to use NHS 111 online or call 111
- NHS: Anaphylaxis
- NHS: Help for suicidal thoughts
- NICE: Cannabis-based medicinal products, NG144
- MHRA: Yellow Card reporting site
- GMC: Information for doctors on cannabis-based products for medicinal use
- CQC: Cannabis-based medicinal products: what CQC expects providers to consider
