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The Royal College of Psychiatrists Improving the lives of people with mental illness


Cannabis and mental health: key facts

More older people in US using marijuana


For many of us, cannabis is a way to relax – 2 million people in the UK smoke it and half of all 16 to 29 year olds have tried it. But research suggests that it can cause problems in some vulnerable people.


How does cannabis work?

When smoked, cannabis from the lungs goes into the blood and is carried to every part of the body. Several chemicals in cannabis bind to receptors in areas of the brain that deal with pleasure, memory, thought, concentration and the awareness of time. There are two main kinds of chemical involved:


  • A group called the cannabinoids, which seem to give you the more pleasant effects - feeling relaxed, happy, sleepy, with colours appearing more vivid and music sounding better.
  • THC, which seems to produce hallucinations, anxiety and paranoia.

These feelings don't usually last long - although as the drug can stay in your system for some weeks, subtle effects can last a few days. Long-term use can make you depressed and make you less motivated.


What is the risk to mental health?


1600 Australian children aged 14 to 15 were studied for seven years. The ones who used cannabis every day were 5 times more likely to become depressed and anxious by the end of the study.


  • If you start smoking cannabis before 15, you are 4 times more likely to develop a psychotic illness.
  • The more cannabis you use, the more likely you are to develop psychosis.

It isn't clear why cannabis use in adolescence seems to have such an effect, but it may be because the brain is still developing.


Is there such a thing as ‘cannabis psychosis’?

Some people seem to get a short period of psychosis that is brought on by cannabis but which stops soon after the cannabis is stopped.


If it's so dangerous, why don't more of my friends get unwell?

Probably because most people don't use cannabis before they are 15 and  they don't go on smoking large amounts. Psychotic illness is quite unusual anyway – only about 1 in 200 people have it at any given time. Most of us probably don't know that many people so, even if cannabis does increase the risk, you aren't likely to notice an ”epidemic” among the people you know.


What about other effects?

  • Education: the connection isn't clear, but regular cannabis use does seem to affect how you do at school or college.
  • Employment: cannabis users are more likely to leave work without permission, spend work time on personal matters or daydream. Regular users report that it has interfered with their work and social life.
  • Driving: a recent study in France showed that cannabis users are more than twice as likely to be the cause of a fatal crash than to be one of the victims.

Is cannabis addictive?

It has some of  the features of addictive drugs – a regular user has to take more and more to get the same effect (tolerance) and can get withdrawal symptoms.


3 out of 4 long-term users get cravings, half become irritable and 7 out of 10 switch to tobacco to try to stay off cannabis.  Many find that they spend much of their life seeking, buying and using it. It is probably about as hard to stop as tobacco.


What about skunk and stronger varieties of cannabis?

The amount of the main active ingredient, THC, in herbal cannabis varies from 1% up to 15%. The newer strains, including skunk, can have up to 20%. On the whole, the newer varieties are probably about 2 or 3 times stronger than those available 30 years ago. They make you relaxed and cheerful more quickly, but also produce more unpleasant effects.


How can I cut down my use of cannabis?

Guidance on cutting down and stopping suggests that you:

  • write down your reasons for wanting to change
  • plan how you will change
  • plan how to cope with withdrawal symptoms
  • have a back-up plan.

You can also work through the leaflet on the FRANK website.


What other support can I get?

What about professional help?

Your GP or practice nurse can put you in touch with a counsellor, support group or NHS substance misuse service. These can help you to stop taking cannabis, to cut down the amount you use and to reduce its impact on your life.  Some now have services especially for cannabis users.


For more in-depth information see our main leaflet: Cannabis and Mental Health

This leaflet reflects the most up-to-date evidence at the time of writing.

Produced by the RCPsych Public Education Editorial Board.

Series Editor: Dr Philip Timms

Reviewed by Dr Ros Ramsay

© January 2015. Due for review: January 2018. Royal College of Psychiatrists. This leaflet may be downloaded, printed out, photocopied and distributed free of charge as long as the Royal College of Psychiatrists is properly credited and no profit gained from its use. Permission to reproduce it in any other way must be obtained from The College does not allow reposting of its leaflets on other sites, but allows them to be linked directly.

For a catalogue of public education materials or copies of our leaflets contact: Leaflets Department, The Royal College of Psychiatrists, 21 Prescot Street, London E1 8BB. Telephone: 020 3701 2552.

Charity registration number (England and Wales) 228636 and in Scotland SC038369.

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