Side Effects of Medical Cannabis
Medical cannabis is legally prescribed in the UK for a range of eligible conditions when conventional treatments have not provided sufficient relief. For many patients, it forms part of a carefully managed, specialist-led treatment plan.
Like all prescribed medicines, medical cannabis side effects are possible. These effects vary widely between individuals and depend on factors such as the type of product prescribed, cannabinoid content, dose, method of administration, and a person’s overall health.
This page provides clear, balanced, and UK-compliant information about medicinal cannabis side effects, and how clinicians work with patients to manage them safely.
Important: Eligibility and suitability for medical cannabis are always assessed on a case-by-case basis by a Medicann clinician.
Understanding Medical Cannabis and its Side Effects
Medical cannabis refers to cannabis-based products for medicinal use (CBPMs) prescribed by a specialist doctor. These products may contain:
CBD (cannabidiol)
THC (tetrahydrocannabinol)
Or a carefully balanced combination of both.
The side effects a patient may experience largely depend on the specific cannabinoids present, their ratios, how the medicine is taken, and the individual’s endocannabinoid system.
Not everyone experiences side effects. Many patients report none or only mild effects that settle as the body adjusts. However, understanding potential risks is an important part of informed consent and safe prescribing.
Common Medical Cannabis Side Effects
Some cannabis side effects can occur with both CBD- and THC-containing medicines, particularly during the early stages of treatment or following dose changes.
General cannabis side effects may include:
Dry mouth
Dry eyes
Fatigue or drowsiness
Mild dizziness or light-headedness
Changes in appetite
Headache.
These effects are usually temporary and often improve as dosing is adjusted or tolerance develops.
CBD Side Effects and CBD Oil Side Effects
CBD is non-intoxicating and does not cause a “high”. As a result, CBD side effects are generally considered mild compared to THC.
Common CBD Oil Side Effects
Patients prescribed CBD-dominant products may experience:
Drowsiness or tiredness
Gastrointestinal discomfort (such as nausea or diarrhoea)
Changes in appetite
Dry mouth
Headache
CBD oil side effects are more likely to occur at higher doses or when starting treatment. Clinicians often begin with a low dose and increase gradually to minimise unwanted effects.
Less Common CBD Side Effects
In rarer cases, some patients report:
Light-headedness
Changes in mood
Sleep disturbance.
CBD can also interact with certain medications by affecting how they are metabolised in the liver. This is why a full medication review is always carried out before prescribing.
THC Side Effects
THC is the psychoactive cannabinoid in medical cannabis. While it may be appropriate for some patients, THC side effects require closer monitoring.
Common THC Side Effects
These can include:
Drowsiness or sedation
Dizziness
Dry mouth and eyes
Short-term memory changes
Slower reaction times.
Because of these effects, patients prescribed THC-containing medicines are advised not to drive unless they meet legal driving requirements and feel unimpaired.
Psychological THC Side Effects
Some people may experience:
Anxiety or nervousness
Low mood
Confusion
Changes in perception.
These effects are more likely at higher doses or in patients who are sensitive to THC. For this reason, clinicians may recommend CBD-dominant products first, particularly for patients new to medical cannabis.
Medicinal Cannabis Side Effects by Product Type
Cannabis Oils
Oils are one of the most commonly prescribed formats. Side effects may develop gradually due to slower absorption.
Possible effects include:
Drowsiness
Digestive discomfort
Dry mouth.
Oils allow for precise dosing, which helps clinicians manage side effects effectively.
Vaporised Flower or Cartridges
Inhaled products act more quickly, meaning side effects can appear sooner.
These may include:
Light-headedness
Throat irritation
Faster onset of THC-related effects.
Your clinician will explain appropriate use and dosing to reduce risk.
Capsules and Lozenges
These formats can cause:
Delayed onset of effects
Digestive upset in some patients.
They may be suitable for those who prefer not to inhale medication.
Less Common but Important Side Effects
While uncommon, some medical cannabis side effects require prompt clinical review.
These may include:
Persistent anxiety or panic
Severe dizziness or fainting
Ongoing nausea or vomiting
Palpitations
Worsening mental health symptoms.
Patients are encouraged to report any concerning effects to their Medicann clinician as soon as possible.
Who Is More Likely to Experience Side Effects?
Side effects can be influenced by:
Sensitivity to cannabinoids
Previous cannabis exposure
Existing medical conditions
Mental health history
Other prescribed medications.
This is why medical cannabis is only prescribed following a comprehensive specialist assessment.
Managing Medical Cannabis Side Effects
Managing medical cannabis side effects is a key part of safe, ongoing care. Medicann clinicians use several strategies to reduce unwanted effects.
Start Low and Go Slow
Most patients begin on a low dose, which is increased gradually. This approach allows the body to adjust and helps identify the lowest effective dose.
Adjusting the Product Type
If side effects occur, clinicians may:
Switch from THC-dominant to CBD-dominant products
Change the CBD:THC ratio
Adjust the format (e.g. oil instead of inhaled medicine).
Timing of Doses
Certain formulas, such as those that are an indica strain, may cause drowsiness. Taking doses in the evening may help reduce daytime drowsiness. Spacing doses can also minimise side effects.
Monitoring and Reviews
Regular follow-ups allow clinicians to assess benefits and side effects, ensuring treatment remains appropriate and safe.
Medical Cannabis Interactions and Safety
Medical cannabis may interact with certain medications, including some antidepressants, pain medicines, and antiepileptic drugs. This is why it is essential to disclose all current medications during your assessment.
Medical cannabis should never be combined with illicit cannabis or altered without medical guidance.
FAQs
Are medical cannabis side effects permanent?
In most cases, side effects are temporary and resolve with dose adjustments or over time.
Are CBD side effects safer than THC side effects?
CBD is generally better tolerated and non-intoxicating, but all medicines can cause side effects in some individuals.
Can side effects be avoided completely?
Not always, but careful prescribing and monitoring significantly reduce risk.
What should I do if I experience side effects?
Contact your Medicann clinician. Do not change or stop your medication without medical advice.
Making an Informed Decision
Understanding the pros and cons and side effects of cannabis helps patients make informed decisions about their care. If you’re considering medical cannabis and want to understand whether it may be suitable for you, check your eligibility or contact our team today. You can also learn more on our FAQ page.
Medicann is the lowest-cost medical cannabis clinic in the UK, offering free consultations for life, no repeat prescription fees, and same-day UK dispatch. Our specialist clinicians are here to guide you through every step, safely and transparently.