Like all prescription medicines, medical cannabis has the potential to cause side effects in some patients.
Side effects are unwanted or unexpected effects that may happen while taking a medicine. Most side effects reported by medical cannabis patients are mild. They often improve over time, or with changes to the dose or product.
This guide outlines the most commonly reported side effects of medical cannabis. It also explains what patients should be aware of.
At a Glance
- Medical cannabis may cause side effects, although not everyone experiences them.
- The most commonly reported side effects include dry mouth, dizziness, sleepiness, fatigue, and headache and most side effects of medical cannabis are mild or moderate.
- Side effects are more common when starting treatment or increasing the dose.
- Patients should report any adverse effects to their prescribing clinic, so treatment can be adjusted if necessary.
What are the Most Common Medical Cannabis Side Effects?
Every patient responds differently to medical cannabis. Side effects can depend on:
- The specific product prescribed
- The quantity of tetrahydrocannabinol (THC) and cannabidiol (CBD)
- The dose and how the medicine is taken
- Age and medical history
- Previous experience with cannabis
Real-world data from the UK Medical Cannabis Registry, which collects anonymised clinical outcomes and safety data from patients prescribed medical cannabis in the UK, shows that approximately 14% of patients report adverse events [1]. The majority of these are mild or moderate.
Patients who had not previously used cannabis were more likely to report side effects when starting treatment.
The most commonly reported side effects include:
- Dry mouth
- Dizziness
- Sleepiness
- Fatigue
- Headache
- Impaired concentration
Curaleaf Clinic patients are advised to log any adverse effects as soon as possible in their Curaleaf Clinic Account under ‘Report’ and to discuss these with their prescriber during consultation so that relevant adjustments can be considered.
THC vs CBD: Cannabinoid-specific Side Effects
Medical cannabis contains naturally occurring compounds called cannabinoids. These interact with the body’s endocannabinoid system, which helps regulate functions such as pain, mood, appetite, and sleep.
The two most widely studied cannabinoids are tetrahydrocannabinol (THC) and cannabidiol (CBD).
THC
THC is the compound most commonly associated with psychoactive effects.
In medical use, the intended purpose is to support symptom management rather than to produce intoxication. However, depending on the dose and individual sensitivity, THC may cause:
- Drowsiness
- Dizziness
- Anxiety
- Temporary confusion
- Impaired coordination
- A feeling of being “high”
Higher doses of THC are more likely to cause these effects. This is particularly the case for patients who are new to cannabis. Prescribing specialists typically aim to increase doses gradually, with the goal of reducing the likelihood of unwanted effects.
Patients with a history of psychosis or certain psychiatric conditions may be advised to avoid or use THC-containing products with particular caution.
CBD
CBD does not typically produce intoxicating effects. It is generally well tolerated, but side effects can still occur. These may include:
- Fatigue
- Stomach or digestive upset
- Changes in appetite
High doses of pharmaceutical-grade CBD (such as products used in epilepsy treatment) have been associated with changes in liver function in some patients. For this reason, clinicians may recommend blood tests where clinically appropriate.
To learn more about the role of different cannabinoids in medical cannabis, you can read our guide on the differences between CBD and THC.
Who Should Take Extra Care with Medical Cannabis?
Medical cannabis may not be suitable for everyone. Before prescribing, specialists carry out a detailed medical assessment to determine whether treatment is appropriate and safe.
Patients should discuss their full medical history with their clinician, particularly if they:
- Have a history of psychosis or schizophrenia
- Have significant heart or cardiovascular conditions
- Have a history of liver problems or abnormal function
- Are pregnant, planning a pregnancy, or breastfeeding
- Have a history of substance misuse
- Take medications that may interact with cannabinoids
- Have significant respiratory (lung) conditions such as severe asthma or COPD
- Are immunocompromised or have a weakened immune system
Your specialist will assess potential risks, discuss suitable product options, and provide ongoing monitoring throughout your treatment.
Driving and Operating Machinery
Medical cannabis may affect alertness, coordination, reaction time, and concentration, particularly when starting treatment or adjusting the dose.
In the UK, it is illegal to drive while impaired by cannabis. Even if you have a valid prescription, you must not drive if you feel affected by your medication.
If you experience drowsiness, dizziness, slowed thinking, or impaired coordination, you should not drive or operate machinery. If you are unsure whether it is safe to drive, speak to your clinician for advice.
Do the Side Effects of Medical Cannabis Differ from Illicit Cannabis?
Illicit cannabis refers to cannabis products obtained outside the UK’s legal medical framework. These products have not been prescribed by a specialist clinician or dispensed by a regulated pharmacy.
Unlike prescribed medical cannabis, illicit products are not subject to standardised manufacturing, laboratory testing, or quality control requirements. As a result, their strength, cannabinoid content, and overall composition may vary significantly.
Our study analysing illicit cannabis in the UK has identified inconsistencies in THC concentration and the presence of contaminants such as pesticides, heavy metals, or microbial impurities. These factors may increase the risk of unexpected side effects or adverse reactions.
Prescribed medical cannabis products supplied through regulated pharmacies are manufactured and tested according to established quality standards. This helps provide greater consistency in dosing and composition.
All patients who have previously sourced cannabis illicitly are counselled to refrain from doing so whilst medical cannabis is also prescribed. This is to avoid the harms associated with illicit products and to allow clinicians to appropriately assess the response to medical cannabis in isolation.
How Does Medical Cannabis Interact with Other Medications?
CBD and THC can both affect how other medicines are metabolised. This can alter their effectiveness and safety. As part of the prescribing process, clinicians carry out a comprehensive review of a patient’s current medications before starting treatment.
Potential interactions can vary depending on the specific medicines involved. A 2024 systematic review of existing research found that medical cannabis may alter the effects of certain prescription medications, including some commonly used to treat conditions such as epilepsy [2].
Because of this, it is essential that patients provide their specialist with accurate information about all medications they are taking, including over-the-counter treatments and supplements. Clinicians will assess potential interactions as part of prescribing decisions and ongoing monitoring.
Communication between a patient’s GP and cannabis prescriber helps support coordinated care.
How Can You Manage Medical Cannabis Side Effects?
Most side effects associated with medical cannabis are mild and can be managed with appropriate clinical support.
If you experience any unwanted effects, the most important step is to inform your prescribing specialist. Early communication allows your clinician to review your treatment and make adjustments where needed. These may include:
- Starting at a lower dose and increasing gradually
- Adjusting the THC and CBD balance
- Modifying the dose or frequency
- Changing the formulation or route of administration
- Stopping the medication in a safe manner
In the UK, suspected side effects can also be reported to the Medicines and Healthcare products Regulatory Agency (MHRA) through the Yellow Card Scheme. This national system collects safety information about medicines to help identify potential risks and improve patient safety.
Your clinic can submit a Yellow Card report on your behalf, or you may choose to report directly via the MHRA website.
Reporting side effects, even if they seem mild, contributes to ongoing safety monitoring and helps improve understanding of medical cannabis treatments.
When Should You Contact Your Clinic?
You should contact your prescribing clinic immediately if you experience:
- Persistent or worsening side effects
- Severe anxiety or mood changes
- Fainting or significant heart palpitations
- Confusion or unusual behavioural changes
- Changes in perception, such as hallucinations
- Any symptoms that feel concerning or unexpected
Do not stop treatment abruptly without medical advice unless you are experiencing a serious reaction. Your clinician can advise you on the safest next steps.
Curaleaf Clinic patients can report side effects easily in their Curaleaf Clinic Online Account under ‘Report’.
If you experience urgent or severe symptoms due to medical cannabis therapy, you should seek help via an Emergency Department or calling 999, as well as contacting your clinic.
NHS 111 now provides mental health support, in addition to physical health, and access to emergency care 24 hours a day, 7 days a week. To access this dial 111.
Conclusion
Side effects are an important consideration when starting any new treatment, including medical cannabis. While some patients may experience unwanted effects, most reported side effects are mild and manageable with appropriate clinical supervision.
Careful prescribing, gradual dose adjustments, and regular follow-up appointments help ensure that treatment is tailored to each patient’s individual needs.
If you are considering treatment or are already receiving care, understanding potential side effects, and knowing when and how to report them, is an important part of managing your treatment safely.
If you are interested in finding out whether a medical cannabis prescription is right for you, Curaleaf Clinic offers an eligibility assessment to determine if you may be suitable for a consultation.
References
[1] Curaleaf Clinic. (n.d.). What does UK research show about medical cannabis patient outcomes? Retrieved February 24, 2026, from https://curaleafclinic.com/what-does-uk-research-show-about-medical-cannabis-patient-outcomes/
[2] Ho, J. J. Y., Goh, C., Leong, C. S. A., Ng, K. Y., & Bakhtiar, A. (2024). Evaluation of potential drug-drug interactions with medical cannabis. Clinical and translational science, 17(5), e13812. https://doi.org/10.1111/cts.13812
