KnowledgeSide effects
Medical cannabis side effects: common, rare and warning signs
The most common side effects of medical cannabis are tiredness, dizziness, dry mouth and nausea. They mainly occur at the start, and some can be influenced by a medical adjustment of the treatment. Which effects are common, which are rare and when you need medical help straight away.
AI-generatedAI-generated420doc Magazine
Knowledge
Contents of this article
Like any effective medicine, medical cannabis can have side effects. The most common are tiredness, dizziness, dry mouth and nausea. They mainly appear at the start of treatment or after the amount has been increased. Some ease after a few days, others can be influenced by a medical adjustment of the treatment.
Less common, but more important, are mental side effects such as anxiety, confusion or psychotic symptoms. Below you can read what the data from Germany shows, who should be particularly careful and what you can do.
What the German BfArM survey shows
From 2017 to 2022, Germany's Federal Institute for Drugs and Medical Devices (BfArM) collected data on people with statutory health insurance (GKV, the public health insurance most people in Germany have) whose cannabis treatment had been approved by their insurer. A total of 16,809 complete data sets were analysed. The results are the best German data on side effects in everyday treatment.
Very common was tiredness (14.9 per cent). Common were dizziness (9.8 per cent), sleepiness (6.0 per cent), nausea (4.9 per cent), dry mouth (4.9 per cent), attention problems (4.3 per cent), increased appetite (4.2 per cent), balance problems (3.1 per cent) and memory problems (3.0 per cent).
According to the analysis, stopping treatment because of side effects was rare. The severity of side effects was not recorded, and the data comes from an online survey of the treating doctors. Keep this in mind when interpreting the figures.
What are the physical side effects of cannabis?
Besides tiredness and dizziness, a faster pulse, palpitations, low blood pressure when standing up, nausea, diarrhoea or constipation and blurred vision can occur. Older people in particular should watch out for the risk of falls.
Smoking flowers instead of vaporizing them also puts strain on the airways. In medical use, vaporizing with a vaporizer is the usual method, see Vaporizing cannabis instead of smoking.
A rare condition after long, very frequent use is cannabinoid hyperemesis syndrome, with recurring severe vomiting. It improves after stopping.
What are the mental side effects of THC?
In the BfArM survey, reports included euphoric mood (1.4 per cent), low mood (1.2 per cent), disorientation (1.7 per cent), hallucinations (0.7 per cent) and delusions (0.4 per cent). Free-text answers added anxiety, inner restlessness and panic attacks.
The risk is higher for young people, for people with a personal or family history of psychosis or bipolar disorder and for people with anxiety disorders. Particular caution also applies to heart and circulation diseases and during pregnancy and breastfeeding, when medical cannabis is usually not prescribed. How THC and CBD differ in their risks is explained in THC vs CBD.
Long-term risks
With long-term use, tolerance can develop and the effect then wears off. Like any psychoactive substance, THC can be addictive. When stopping after longer use, sleep problems, irritability and restlessness can occur for a while.
That is why every treatment includes a regular review: do the benefits still outweigh the side effects?
What influences the side effects
Active ingredient content and the ratio of THC to CBD, amount, form of the medicine, time of use, previous experience, age, other illnesses and other medicines. Sedatives, sleeping pills, opioids and alcohol can increase tiredness and dizziness. Always tell your doctor and your pharmacy (Apotheke) about every medicine you take.
Many side effects can be avoided if the dose is adjusted slowly. Your doctor sets the amount and the steps; do not change them yourself.
What you can do about side effects
Write down when a side effect occurs and how strong it is, ideally in a treatment diary. Do not change anything on your own, but talk to your doctor. A smaller amount, a different time of use or a different composition often helps.
Do not drive or operate machinery if you could be impaired, especially during the adjustment phase, after changes to the amount and together with alcohol or sedatives. Even if you feel fine, your reactions can be slower. Ask your doctor when you can drive again. The rules on German roads are explained in the guide Driving with a cannabis prescription.
Warning signs: when you need help immediately
Chest pain, severe palpitations, fainting, a seizure, persistent vomiting, confusion, hallucinations, feeling persecuted or thoughts of harming yourself. In these cases: stop using it, do not drive yourself and get medical help immediately, in an emergency by calling 112, the emergency number in Germany.
Frequently asked questions
What is the most common side effect of medical cannabis?
Tiredness. In the German BfArM survey it occurred in 14.9 per cent of patients.
Do the side effects go away?
Many ease after a few days once the body has got used to the medicine, or improve after the treatment has been adjusted.
Does CBD have fewer side effects than THC?
CBD does not cause a high, but it can cause tiredness, diarrhoea and changes in liver values and can interact with other medicines.
Can you become addicted to medical cannabis?
Yes, the risk exists. It is kept lower through medical supervision, the lowest possible amounts and regular reviews.
What should I do if the effect is too strong?
Sit or lie down calmly, drink water, take nothing more and do not drive. In case of chest pain, impaired consciousness or ongoing panic, call 112.
Sources
- PTAheute: Welche Nebenwirkungen haben Cannabis-Arzneimittel? (BfArM/PEI, Bulletin zur Arzneimittelsicherheit 3/2022, in German)
- Whiting PF et al. (2015): Cannabinoids for Medical Use: A Systematic Review and Meta-analysis. JAMA
- Volkow ND et al. (2014): Adverse Health Effects of Marijuana Use. N Engl J Med
420doc editorial team · Published on 26 October 2026 · Updated on 26 October 2026
This article is for general information only and does not replace individual medical advice. How we review our content
Read more
All articles
CBD vs THC: the difference explained simply
THC is the intoxicating main active ingredient of the cannabis plant, CBD does not get you high. How both work in the body, which risks they carry, what applies to CBD products in Germany and why CBD does not automatically make THC safer.
Vaporizing cannabis instead of smoking: how to use a vaporizer for medical flowers
When vaporizing, cannabis flowers are heated so that the active ingredients are released without the plant material burning. This produces fewer combustion products than smoking. How a vaporizer works, what matters when using it and what it cannot do.
How long does THC stay in your system? Blood, urine, saliva and hair
How long THC can be detected depends on the test and on how often someone uses cannabis: in blood usually hours, in urine days to weeks, in hair months. With tables on detection windows, limits and the difference between THC, 11-OH-THC and THC-COOH.
Well informed. Factual, with sources.
You will find more background in the magazine and the glossary. Whether a treatment suits you can only be clarified in a personal conversation with a doctor.
To the glossary