What we know about cannabis
The largest study so far is the European EU-GEI study, which I took part in. At 11 sites in Europe and Brazil we compared 901 people with a first psychosis with 1,237 people without psychosis. Those who used cannabis daily were more than 3 times as likely to have a psychosis as those who never used it. With daily use of high-potency cannabis, with 10 per cent THC or more, that was almost 5 times. In Amsterdam, where high-potency cannabis is available everywhere, an estimated half of the new cases of psychosis were linked to high-potency cannabis.
What does that mean for you? Most people who use cannabis never have a psychosis. The research shows that strong cannabis, used often and from an early age, raises the chance that your experience starts to shift. Those who start around the age of 15 run a greater risk than those who start later. And if you are already sensitive to seeing meaning, to suspicion or to stress, you notice it sooner.
Nitrous oxide
Nitrous oxide is also known as laughing gas. In the Netherlands, where PsychosisNet comes from, possession and sale for recreational use have been banned since 1 January 2023. The rules differ from country to country. The health risk lies mainly in using a lot and often. Nitrous oxide inactivates vitamin B12, and a shortage of it can cause nerve damage: tingling, loss of strength, difficulty walking. With heavy use, confusion, suspicion and psychosis have also been described. Do you have tingling hands or feet after using nitrous oxide? See your doctor and say what you have used.
Ketamine
Ketamine is an anaesthetic that is also used at parties. It gives an experience of coming loose: from your body, from your surroundings, sometimes from time. Researchers use ketamine precisely because in volunteers it temporarily produces experiences that resemble psychosis. In most people this fades when the drug has worn off. With frequent use, the detachment and the suspicion can linger, and your bladder can be damaged.
Why it goes wrong for one person and not for another
Psychosis rarely has a single cause. It is an accumulation: little sleep, stress, loneliness, bad things you have been through, a sensitive profile, and a substance on top of that. A substance amplifies what is already going on. If you are sensitive to meaning, you experience even more meaning under the influence. If you are already suspicious, you become more suspicious. That is how your friend can smoke cannabis every day without problems, while after a few months you no longer sleep and see signs everywhere. Read more about the causes of psychosis and about drug use and psychosis.
Signs that your experience is starting to shift
- You sleep worse or much less.
- You see meaning in chance things: a number plate, a song, something a person in the street says.
- You feel watched or talked about.
- Sounds are different, or you hear something that others do not hear.
- You withdraw, because others no longer seem to understand you.
What you can do
- Use less, and less strong. You do not have to stop all at once; cutting down already helps.
- Sleep first. Sleep is often the first thing to go and the first thing you can get back.
- Tell someone you trust. Not to ask permission, but so that someone keeps an eye on things with you.
- Know that stopping after a long period of daily cannabis use can feel restless for a few weeks, with poor sleep and irritability. That usually fades.
Jim van Os
Sources: Di Forti M et al. (2019). The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI). The Lancet Psychiatry, 6(5), 427-436. Government of the Netherlands (2022). Ban on nitrous oxide from 1 January 2023.
Do you need help now?
PsychosisNet offers information. We do not offer a chat, a helpline or crisis support. If you or someone else is in immediate danger, call the emergency number in your country. If you are thinking about suicide, or you need to talk to someone now, see Need help now? for helplines by country.
Prof. dr. Jim van Os, Chair Division Neuroscience, Utrecht University Medical Centre. Jim works at the interface of ‘hard’ brain science, health services research, art and subjective experiences of people with ‘lived experience’ in mental healthcare.
Jim has been appearing on the Thomson-Reuter Web of Science list of ‘most influential scientific minds of our time’ since 2014. In 2014 he published his book ‘Beyond DSM-5‘, and in 2016 the book ‘Good Mental Health Care’. In 2026 he published 'Wat psychisch lijden is'.
Want to read more?
Blogs by Jim van Os can be read here
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