Medical Cannabis Side Effects — What UK Patients Need to Know

Medical cannabis is a prescribed medicine. Like all medicines, it can cause side effects. Understanding what to expect — and how to manage it — helps you get the most from your treatment while staying safe.

Common Side Effects of Medical Cannabis

Most side effects are mild, dose-related and reduce as your body adjusts. The most frequently reported include:

• Dry mouth — cannabinoids reduce saliva production. Staying well hydrated and chewing sugar-free gum can help.
• Drowsiness and fatigue — particularly common with indica-dominant flower and higher-THC products. Many patients take their evening dose specifically for this effect.
• Dizziness or light-headedness — usually brief, especially when standing quickly. Starting at a low dose reduces this.
• Increased appetite — THC stimulates appetite hormones. This can be therapeutic for some patients (e.g. those undergoing chemotherapy) but unwanted for others.
• Short-term memory effects — THC temporarily affects working memory. Avoid cognitively demanding tasks immediately after dosing until you know how you respond.
• Mild anxiety or heightened awareness — more common with high-THC products or when taking too large a dose too quickly.

THC Side Effects vs CBD Side Effects

Most notable side effects come from THC (tetrahydrocannabinol), the psychoactive component. CBD (cannabidiol) has a milder profile:

THC-dominant products are more likely to cause psychoactive effects, appetite changes, drowsiness and memory interference. They are also the products most associated with "feeling high" — an effect that is not the goal of medical treatment but which may occur, especially early on.

CBD-dominant products (including CBD oils prescribed at specialist clinics) rarely cause psychoactive effects. Some patients report mild fatigue at high doses. CBD can interact with certain medications — notably blood thinners (warfarin) and some anti-epileptic drugs — so always inform your prescriber of your full medication list.

If you are unsure which type of product you have been prescribed, check your patient information leaflet or contact your clinic. More background on product types is in our flower vs oil guide.

Your First Dose — What to Expect

The first time you use a prescribed cannabis medicine, treat it cautiously regardless of any prior experience with cannabis. Prescribed products are standardised and often more potent than street cannabis, and the delivery method (vaporiser, oil, capsule) changes onset times significantly.

• Take your first dose at home, in a safe environment.
• Do not drive or operate machinery for at least 4–6 hours after dosing, and longer if you feel any impairment.
• Have someone with you if possible, particularly for the first few uses.
• Note how you feel — your clinic will ask at your follow-up appointment.

Your prescriber will set a starting dose intentionally low. This is called "start low, go slow" and is standard practice across UK licensed clinics.

How to Adjust Your Dose Safely

Never adjust your prescribed dose without speaking to your specialist first. This applies to both increasing and decreasing. Self-adjusting can:

• Increase the risk of side effects if you increase without guidance
• Cause withdrawal-like symptoms (disturbed sleep, irritability) if you stop abruptly after extended use
• Affect your prescription legality — you are licensed for the specific dose and product on your prescription

If you feel your current dose is not effective or is causing problems, contact your clinic. Dose reviews are routine at follow-up appointments, which typically occur every 1–3 months.

Things Not to Do

These combinations carry specific risks for medical cannabis patients:

• Alcohol — both alcohol and THC impair coordination and judgment. Combined, the effects are amplified unpredictably. Avoid alcohol on the same day as dosing, especially early in treatment.
• Driving — UK law sets a blood-THC limit of 2 µg/L. This can be exceeded for many hours after dosing. See our driving guide for full detail.
• Changing products without advice — switching from oil to flower, or to a different strain, changes your effective dose. Always discuss with your prescriber.
• Combining with recreational cannabis — this is illegal and makes dose management impossible.
• Using someone else's prescription — it is a criminal offence to possess a Schedule 2 drug without your own valid prescription.

When to Contact Your Clinic Urgently

Most side effects are manageable, but some warrant prompt medical attention:

• Significant chest pain or palpitations
• Severe panic attack or psychotic symptoms (paranoia, hearing voices, dissociation)
• Seizure or loss of consciousness
• Severe allergic reaction (rash, swelling, breathing difficulty)
• Symptoms of cannabinoid hyperemesis syndrome (cyclical vomiting — rare but serious with very heavy use)

If you experience a medical emergency, call 999. For urgent non-emergency concerns, contact your prescribing clinic's patient support line. Details will be in your welcome documentation.

For a broader overview of conditions treated with medical cannabis, visit our patient FAQ.

Find a UK Clinic

The following licensed UK clinics cover the conditions discussed in this guide. All offer online consultations available UK-wide.

• Lyphe Clinic — from £99
• Releaf — from £79
• Mamedica — from £95
• Alternaleaf — online UK-wide
• Zerenia — from £99

Compare all 9 licensed prescribers on the UK clinic comparison page.

Frequently Asked Questions