Medical Cannabis Side Effects: What UK Patients Should Know
Medical Cannabis Side Effects: What UK Patients Should Know
Like any prescription medicine, medical cannabis can cause unwanted effects. These vary between people and can depend on the product, dose, formulation and balance of cannabinoids such as THC and CBD.
Common medical cannabis side effects can include dizziness, tiredness, dry mouth, nausea and changes in concentration. THC-containing medicines can also cause intoxication and, in some people, changes in anxiety, mood or perception.
Experiencing a side effect does not necessarily mean that medical cannabis treatment has to stop. However, new, persistent or concerning symptoms should be discussed with the clinical team responsible for your prescription.
In this guide, we look at medical cannabis side effects, how THC and CBD differ, what UK patient data tells us about safety, what may happen when treatment begins and when you should seek medical advice.
Contents
What Are the Most Common Medical Cannabis Side Effects?
Medical cannabis side effects can include dizziness, tiredness, dry mouth, nausea, changes in appetite, reduced concentration and changes in mood or perception.
The exact effects vary because medical cannabis is not one single medicine. Different prescribed products contain different concentrations of tetrahydrocannabinol (THC), cannabidiol (CBD) and other compounds.
The NHS guidance on medical cannabis lists possible side effects including:
- Dizziness
- Feeling very tired
- Feeling sick
- Diarrhoea
- Changes in appetite
- Weakness
- Changes in behaviour or mood
- Feeling intoxicated or “high”
- Hallucinations
Not everyone experiences side effects, and the likelihood and severity can differ substantially between individuals.
Why Does Medical Cannabis Cause Side Effects?
Side effects occur because cannabinoids interact with biological systems throughout the body.
THC primarily produces its characteristic effects through cannabinoid receptors, including CB1 receptors that are widely distributed throughout the central nervous system.
These receptors are involved in processes including memory, appetite, movement and perception. This helps explain why THC can produce therapeutic effects while also affecting concentration, coordination, appetite or mood.
CBD works differently and is not intoxicating, but it can still cause side effects and can affect the way some other medicines are metabolised.
The effects also depend on dose. A medicine that is well tolerated at one dose may produce unwanted effects following an increase, which is one reason medical cannabis treatment is individually prescribed and monitored.
THC Side Effects
THC is the main intoxicating cannabinoid in cannabis and is responsible for many of the side effects associated with changes in alertness, perception and coordination.
Possible THC side effects can include:
- Dizziness
- Drowsiness
- Dry mouth
- Reduced concentration
- Changes in memory
- Feeling intoxicated
- Changes in coordination
- Anxiety or paranoia in some people
- Changes in heart rate
The current Summary of Product Characteristics for Sativex, a licensed medicine containing both THC and CBD, identifies dizziness and fatigue among its most frequently reported adverse reactions.
It also reports effects including somnolence, disturbances in attention, memory impairment, dry mouth, nausea and changes in mood among possible adverse reactions.
Sativex is one specific licensed medicine, so its adverse-event profile should not simply be assumed to apply identically to every prescribed cannabis product. However, it provides useful controlled clinical information about the potential effects of THC and CBD.
CBD Side Effects
CBD does not produce the intoxicating effects associated with THC, but that does not mean it is completely free from side effects.
Depending on the medicine and dose, CBD can be associated with effects such as:
- Tiredness or drowsiness
- Diarrhoea
- Changes in appetite
- Gastrointestinal discomfort
CBD can also affect enzymes involved in processing other medicines. This means a potential drug interaction can sometimes be more important than an immediately noticeable side effect.
Tell your prescribing clinician about all prescription medicines, over-the-counter products and supplements you take so that potential interactions can be considered.
Can Medical Cannabis Make You Feel Tired?
Yes. Tiredness, fatigue and drowsiness are recognised medical cannabis side effects.
This is particularly important when treatment is first started or when a dose or product changes, because you may not yet know exactly how the medicine affects you.
Real-world UK data supports this. An observational analysis of patients in the UK Medical Cannabis Registry found fatigue, somnolence and lethargy among the most frequently reported adverse events.
If your prescription makes you unusually tired or sleepy, discuss this with your clinical team — particularly if it affects work, driving or other activities requiring concentration.
Can Medical Cannabis Cause Dizziness?
Yes. Dizziness is a recognised side effect of medical cannabis and may be particularly noticeable when treatment begins or the dose changes.
Current prescribing information for Sativex notes that mild or moderate dizziness is commonly reported and occurs most frequently during the first few weeks of treatment.
Feeling dizzy can increase the risk of falls and may make activities such as driving unsafe.
If dizziness is persistent, severe or affecting your ability to function normally, contact your clinical team rather than simply trying to tolerate it.
Why Does Cannabis Cause Dry Mouth?
Dry mouth is another commonly reported effect associated with cannabinoids.
Cannabinoid receptors are present in the systems involved in saliva production, and cannabinoid activity can temporarily reduce salivary secretion.
Dry mouth is included among the recognised adverse effects in clinical information for cannabinoid medicines and has also been reported in UK medical cannabis observational data.
If dry mouth becomes persistent or troublesome, mention it during your review. Ongoing dry mouth can be uncomfortable and may also affect oral health.
Can Medical Cannabis Cause Anxiety?
THC can increase anxiety in some people, particularly depending on dose and individual sensitivity.
Cannabis does not affect everybody’s mood in the same way. Some people may feel relaxed, while others can experience nervousness, anxiety, panic-like symptoms or paranoia.
This is one reason mental health history forms an important part of medical cannabis assessment.
More significant psychiatric effects are possible but less common. The NHS identifies hallucinations among potential adverse effects of medical cannabis, while Sativex clinical data includes uncommon reports of hallucinations and paranoia.
If your medication causes a significant or unexpected change in anxiety, mood, perception or behaviour, contact your clinical team.
Can Medical Cannabis Affect Memory and Concentration?
THC-containing medical cannabis can temporarily affect attention, memory, reaction time and concentration.
How noticeable this is varies according to the person, product, dose, tolerance and timing.
Sativex prescribing information includes disturbance in attention and memory impairment among reported adverse effects.
These effects become particularly important when carrying out activities that require alertness or coordination.
Do not drive or operate machinery if your medication is making you drowsy, dizzy, confused, noticeably intoxicated or unable to concentrate normally.
Does Medical Cannabis Interact With Other Medicines?
Yes, cannabinoids can interact with some medicines, although the significance depends on the individual medicine, cannabinoid, dose and patient.
THC and CBD are metabolised through liver enzyme systems that are also involved in processing many prescription medicines. CBD in particular can affect some of these pathways.
Interactions can potentially alter the amount of another medicine in the body or increase overlapping effects such as drowsiness and dizziness.
We recently explored this in detail in our guide to medical cannabis and antidepressants.
Always give your medical cannabis clinician an accurate list of the medicines, supplements and other products you use. Do not independently stop another prescribed medicine because you are starting medical cannabis.
What Does UK Patient Data Tell Us About Side Effects?
One useful source of real-world information is the UK Medical Cannabis Registry, which collects outcomes and adverse-event data from patients prescribed cannabis-based medicinal products.
In a large observational analysis, commonly reported adverse events included:
- Fatigue
- Somnolence
- Dry mouth
- Lethargy
- Headache
The study also found that patients who were cannabis-naïve before treatment were more likely to report adverse events than people who were already using cannabis at baseline.
This is useful real-world evidence, but there is an important limitation: registry studies are observational. They cannot prove that every reported event was caused by the prescribed cannabis medicine.
They are best used alongside controlled trials, prescribing information and ongoing clinical monitoring rather than as a precise prediction of what an individual patient will experience.
What About Long-Term Medical Cannabis Side Effects?
The long-term safety evidence for many cannabis-based medicinal products is still developing.
This is particularly important because medical cannabis is not one standardised medicine. Products vary considerably in THC and CBD concentration, formulation and prescribed dose.
The NHS England guidance on cannabis-based products for medicinal use notes that potential harms associated with long-term use have not yet been fully studied.
Regular exposure to THC can also lead to tolerance and dependence in some people.
Tolerance means that the body adapts to repeated exposure, while dependence means stopping or significantly reducing regular THC use can result in withdrawal symptoms.
We cover this separately in our guide to cannabis withdrawal symptoms and the typical withdrawal timeline.
These possibilities are another reason prescribed treatment should be reviewed over time rather than considered completely risk-free once someone has found a medicine that works for them.
What Should You Do if You Experience Medical Cannabis Side Effects?
If you develop a new or troublesome side effect while taking prescribed medical cannabis, contact the clinical team responsible for your treatment.
Try to provide useful information such as:
- What symptom you experienced
- When it began
- Which prescribed product you had taken
- How much you had taken
- When you took it
- How long the effect lasted
- Whether anything had recently changed
- Any other medicines or substances you had used
Do not make substantial changes to prescribed treatment without discussing them with your clinical team unless you require urgent medical attention.
NICE recommends that the efficacy and safety of cannabis-based medicinal products are monitored and that adverse reactions form part of ongoing treatment review.
Your clinician can then decide whether your treatment needs reviewing according to your individual circumstances.
When Should You Seek Urgent Medical Help?
Most side effects do not require emergency treatment. However, some symptoms should not simply be monitored at home.
Seek urgent medical help if you become seriously unwell or experience symptoms such as:
- Loss of consciousness
- Severe confusion or disorientation
- Severe hallucinations or psychotic symptoms
- A severe or concerning change in mental state
- Thoughts of harming yourself or somebody else
- Another symptom that appears to require immediate medical attention
If you are unsure whether a symptom needs urgent attention, NHS 111 can provide guidance. In a medical emergency, call 999.
Do not drive yourself for medical attention if you feel intoxicated, dizzy, drowsy, confused or otherwise impaired.
Can You Drive if You Have Medical Cannabis Side Effects?
You must not drive if medical cannabis is impairing your ability to drive safely.
Side effects such as dizziness, drowsiness, reduced concentration, blurred vision, impaired coordination or feeling intoxicated can make driving unsafe.
A statutory medical defence may apply to prescribed THC in certain circumstances when the medicine has been lawfully prescribed and taken as directed. However, this does not provide a defence for driving while impaired.
You should be particularly cautious when beginning treatment, changing products or changing dose because you may not yet know how your medication affects you.
For a fuller explanation, read our guide to medical cannabis, THC and driving in the UK.
Can Medical Cannabis Side Effects Be Reported?
Yes. Suspected side effects from medicines can be reported through the UK’s MHRA Yellow Card scheme.
The Yellow Card scheme collects reports from both healthcare professionals and members of the public to help identify and monitor potential medicine safety issues.
You do not need to prove that your medicine caused the reaction before reporting a suspected side effect.
You can find more information through the MHRA Yellow Card scheme.
Reporting a suspected reaction does not replace medical advice. Contact your clinical team if you are concerned about how your prescribed medication is affecting you.
Medical Cannabis Side Effects FAQs
What Are the Most Common Medical Cannabis Side Effects?
Commonly reported medical cannabis side effects include tiredness, dizziness, dry mouth, nausea and changes in concentration. The effects vary according to the individual, product, cannabinoid content and dose.
Can Medical Cannabis Make You Feel Tired?
Yes. Fatigue, drowsiness and somnolence are recognised effects of cannabis-based medicines and have also been reported in UK medical cannabis observational data. Tell your clinical team if tiredness is persistent or interferes with everyday activities.
Can Medical Cannabis Make You Dizzy?
Yes. Dizziness is a recognised side effect and can be particularly noticeable when treatment begins or the dose changes. Do not drive if dizziness is affecting you.
Why Does Medical Cannabis Give Me a Dry Mouth?
Cannabinoids can affect the systems involved in saliva production, which can temporarily reduce saliva and produce a dry sensation in the mouth. If it becomes persistent or troublesome, discuss it with your clinician.
Can Medical Cannabis Cause Anxiety?
THC can increase anxiety in some people, depending on factors including dose and individual sensitivity. Significant anxiety, panic, paranoia or other changes in mental state should be discussed with your clinical team.
Can Medical Cannabis Cause Paranoia?
THC can cause paranoia or other changes in perception in some people. These effects are not experienced by everyone. If you develop significant paranoia, hallucinations or another concerning change in your mental state, seek appropriate medical advice.
Does CBD Have Side Effects?
Yes. Although CBD is not intoxicating, it can cause effects such as tiredness, gastrointestinal symptoms and changes in appetite. CBD can also interact with some medicines by affecting the enzymes involved in drug metabolism.
Do Medical Cannabis Side Effects Go Away?
Some unwanted effects may become less noticeable as treatment continues, but this does not happen in every case. The appropriate response depends on the symptom, severity, product and individual patient. Persistent or concerning effects should be discussed with your prescribing team.
Are Medical Cannabis Side Effects Worse When You First Start?
Some effects may be more noticeable when treatment begins because you do not yet know how you will respond. Current Sativex prescribing information, for example, reports dizziness most frequently during the first few weeks of treatment. Your experience with another prescribed cannabis medicine may be different.
Can You Become Tolerant to Medical Cannabis?
Regular THC exposure can result in tolerance in some people. This means the body adapts to repeated exposure. If you feel your prescribed treatment is becoming less effective, discuss this with your clinician rather than independently increasing the amount you use.
Can You Become Dependent on Medical Cannabis?
Dependence can occur with regular THC exposure. This does not mean everyone prescribed medical cannabis will become dependent. If you are concerned about tolerance, dependence or withdrawal, discuss it with your prescribing clinician.
Can Medical Cannabis Interact With Other Medication?
Yes. THC and CBD can interact with some medicines, including through liver enzymes involved in drug metabolism. Other medicines may also have overlapping effects such as sedation. Tell your clinician about all medicines and supplements you take.
Should I Stop Medical Cannabis if I Get Side Effects?
Do not make substantial changes to prescribed treatment based only on general online information. Contact your clinical team if you experience new or troublesome effects so they can advise you according to your prescription and individual circumstances. Seek urgent medical help for serious symptoms.
The Bottom Line
Medical cannabis side effects are possible, just as they are with other prescription medicines.
Common effects can include tiredness, dizziness, dry mouth, nausea and changes in concentration. THC can also produce intoxication and may affect mood, anxiety, memory, coordination and perception in some people.
The likelihood of experiencing an unwanted effect depends on several factors, including your prescription, THC and CBD content, dose, other medication and individual sensitivity.
UK real-world evidence is continuing to develop, while NICE recommends ongoing monitoring of both the effectiveness and safety of cannabis-based medicinal products.
If you are already prescribed medical cannabis and experience a new or concerning effect, speak to the clinical team responsible for your treatment rather than making significant changes yourself.
Considering Medical Cannabis?
Medical cannabis has been legally available on prescription in the UK since 2018. Treatment is not suitable for everyone and requires an individual clinical assessment.
At CB1 Medical, your medical history, previous treatments, current medicines and individual risk factors are considered when assessing whether treatment may be appropriate.
Different formulations are available, including prescribed flower used with a dry herb vaporiser, oils and pastilles. You can learn more in our guide to medical cannabis flower, oils and pastilles.
Curious if you’re eligible? You can check your eligibility online to see whether you may be suitable for a specialist consultation.
