Cannabis and mental health
Two million people in the UK smoke cannabis. Half of all 16 to
29 year olds have tried it at least once. In spite of government
warnings about health risks, many people see it as a harmless
substance that helps you to relax and ‘chill’ – a drug that, unlike
alcohol and cigarettes, might even be good for your physical and
mental health. On the other hand, recent research has suggested
that it can be a major cause of psychotic illnesses in
those who are genetically vulnerable.
This leaflet looks at the research on the
effects of cannabis use and mental health and is for anyone who is
concerned about the issue. We hope that this will help people to
make informed choices about using – or not using –
What is cannabis?
Cannabis sativa and cannabis
indica are members of the nettle family that have grown wild
throughout the world for centuries. Both plants have been used for
a variety of purposes including hemp to make rope and textiles, as
a medical herb and as the popular recreational drug.
The plant is used as:
- The resin – a brown/black lump, known
as bhang, ganja, hashish, resin etc;
- Herbal cannabis – made up of the dried
flowering tops and variable amounts of dried leaves - known as
grass, marijuana, spliff, weed etc.
Skunk refers to a range of stronger
types of cannabis, grown for their higher
concentration of active substances. The name refers to the
pungent smell they give off while growing. They can be grown
either under grow-lights or in a greenhouse, often using hydroponic
(growing in nutrient rich liquids rather than soil) techniques.
There are hundreds of other varieties of cannabis with exotic names
such as AK-47 or Destroyer.
Street cannabis can come in a wide variety of
strengths, so it is often not possible to judge exactly what
is being used in any one particular session.
How is it used?
Most commonly, the resin or the dried leaves
are mixed with tobacco and smoked as a ‘spliff’ or ‘joint’. The
smoke is inhaled strongly and held in the lungs for a number of
seconds. It can also be smoked in a pipe, a water pipe, or
collected in a container before inhaling it - a 'bucket'. It can be
brewed as tea or cooked in cakes.
More than half of its psychologically active
chemical ingredients are absorbed into the blood when smoked.
These compounds tend to build up in fatty tissues throughout the
body, so it takes a long time to be excreted in the urine. This is
why cannabis can be detected in urine up to 56 days after it has
last been used.
What is its legal status in the UK?
Cannabis was re-classified in January 2009
and is now a Class B drug under the Misuse of Drugs Act, 1971.
The maximum penalties are:
- For possession: 5 years prison
sentence or an unlimited fine, or both
- For dealing/supplying:14 year prison
sentence or an unlimited fine, or both.
Young people in possession of cannabis
A young person found to be in possession of
cannabis will be:
- Taken to a police station
- Given a reprimand, final warning or
charge, depending on the offence.
After one reprimand, a further offence will
lead to a final warning or charge.
After a final warning:
- The young person must be referred to a
Youth Offending Team to arrange a rehabilitation programme.
- A further offence will lead to a criminal
Adults in possession of cannabis
This will usually result in a warning and
confiscation of the drug. Some cases may lead to arrest and either
caution or prosecution, including:
- repeat offending
- smoking in a public place
- threatening public order.
How does it work and what is the chemical make-up of
There are about 400 chemical compounds in an
average cannabis plant. The four main compounds are called
delta-9-tetrahydrocannabinol (delta-9-THC), cannabidiol,
delta-8-tetrahydrocannabinol and cannabinol. Apart from
cannabidiol (CBD), these compounds are psychoactive, the strongest
one being delta-9-tetrahydrocannabinol. The stronger varieties
of the plant contain little cannabidiol (CBD), whilst the
delta-9-THC content is a lot higher.
When cannabis is smoked, its
compounds rapidly enter the bloodstream and are
transported directly to the brain and other parts of the
body. The feeling of being ‘stoned’ or ‘high’ is caused
mainly by the delta-9-THC binding to cannabinoid receptors in
the brain. A receptor is a site on a brain cell where certain
substances can stick or “bind” for a while. If this happens, it has
an effect on the cell and the nerve impulses it produces.
Curiously, there are also cannabis-like substances produced
naturally by the brain itself – these are called
Most of these receptors are found in the parts
of the brain that influence pleasure, memory, thought,
concentration, sensory and time perception. Cannabis compounds
can also affect the eyes, the ears, the skin and the
What are its effects?
A ‘high’ - a sense of relaxation, happiness,
sleepiness, colours appear more intense, music sounds better.
Around 1 in 10 cannabis users have unpleasant
experiences, including confusion, hallucinations, anxiety and
paranoia. The same person may have either pleasant or unpleasant
effects depending on their mood and circumstances. These
feelings are usually only temporary – although as the drug can
stay in the system for some weeks, the effect can be more
long-lasting than users realise. Long-term use can have a
depressant effect, reducing motivation.
Education and learning
There have also been suggestions that cannabis
may interfere with a person's capacity to:
- organise information
- use information
This effect seems to last several
weeks after use, which can cause particular problems for
However, a large study in New Zealand followed
up 1265 children for 25 years. It found that cannabis use in
adolescence was linked to poor school performance, but that there
was no direct connection between the two. It looked as though it
was simply because cannabis use encouraged a way of life that
didn't help with schoolwork.
It seems to have a similar effect on people at
work. There is no evidence that cannabis causes specific
health hazards. But users are more likely to leave work without
permission, spend work time on personal matters or simply daydream.
Cannabis users themselves report that drug use has interfered
with their work and social life.
Of course, some areas of work are more
demanding than others. A review of the research on the effect of
cannabis on pilots revealed that those who had used cannabis made
far more mistakes, both major and minor, than when they had not
smoked cannabis. As you can imagine, the pilots were tested in
flight simulators, not actually flying... The worst effects
were in the first four hours, although they persisted for at least
24 hours, even when the pilot had no sense at all of being 'high'.
It concluded "Most of us, with this evidence, would not want to fly
with a pilot who had smoked cannabis within the last day or
What about driving?
In New Zealand, researchers found that those
who smoked regularly, and had smoked before driving, were more
likely to be injured in a car crash. A recent study in France
looked at over 10,000 drivers who were involved in fatal car
crashes. Even when the influence of alcohol was taken into account,
cannabis users were more than twice as likely to be the cause of a
fatal crash than to be one of the victims. So - perhaps most of us
would also not want to be driven by somebody who had smoked
cannabis in the last day or so.
Mental health problems
There is growing evidence that people
with serious mental illness, including depression and psychosis,
are more likely to use cannabis or have used it for long periods of
time in the past. Regular use of the drug has appeared to
double the risk of developing a psychotic episode or long-term
schizophrenia. However, does cannabis cause depression and
schizophrenia or do people with these disorders use it as a
Over the past few years, research has strongly suggested that
there is a clear link between early cannabis use and later mental
health problems in those with a genetic vulnerability - and that
there is a particular issue with the use of cannabis by
A study following 1600 Australian
school-children, aged 14 to 15 for seven years, found that while
children who use cannabis regularly have a significantly higher
risk of depression, the opposite was not the case - children who
already suffered from depression were not more likely than anyone
else to use cannabis. However, adolescents who used cannabis daily
were five times more likely to develop depression and anxiety
in later life.
Three major studies followed large numbers of
people over several years, and showed that those people who use
cannabis have a higher than average risk of developing
schizophrenia. If you start smoking it before the age of 15, you
are 4 times more likely to develop a psychotic disorder by the time
you are 26. They found no evidence of self-medication. It seemed
that, the more cannabis someone used, the more likely they
were to develop symptoms.
Why should teenagers be particularly
vulnerable to the use of cannabis? No one knows for certain, but it
may be something to do with brain development. The brain is still
developing in the teenage years – up to the age of around 20, in
fact. A massive process of ‘neural pruning’ is going on. This is
rather like streamlining a tangled jumble of circuits so they can
work more effectively. Any experience, or substance, that affects
this process has the potential to produce long-term psychological
Recent research in Europe, and in the
UK, has suggested that people who have a family background of
mental illness – and so probably have a genetic vulnerability
anyway - are more likely to develop schizophrenia if they use
cannabis as well.
Physical health problems
The main risk to physical health from cannabis is probably from
the tobacco that is is often smoked with.
Is there such a thing as ‘cannabis
Recent research in Denmark suggests that yes,
there is. It is a short-lived psychotic disorder that seems to be
brought on by cannabis use but which subsides fairly quickly once
the individual has stopped using it. It's quite unusual though – in
the whole of Denmark they found only around 100 new cases per
However, they also found that:
- Three quarters had a different psychotic disorder diagnosed
within the next year.
- Nearly half still had a psychotic disorder 3 years later.
So, it also seems probable that nearly half of
those diagnosed as having cannabis psychosis are actually showing
the first signs of a more long-lasting psychotic disorder, such as
schizophrenia. It may be this group of people who are particularly
vulnerable to the effects of cannabis, and so should probably avoid
it in the future.
Is cannabis addictive?
It has some of the features of addictive drugs
- tolerance – having to take more and more to get the same
- withdrawal symptoms. These have been shown in heavy users and
- decreased appetite
- sleep difficulty
- weight loss
- aggression and/or anger
- strange dreams. .
These symptoms of withdrawal produce about
the same amount of discomfort as withdrawing from tobacco.
For regular, long-term users:
- 3 out of 4 experience cravings;
- half become irritable;
- 7 out of 10 switch to tobacco in an
attempt to stay off cannabis.
The irritability, anxiety and problems with
sleeping usually appear 10 hours after the last joint, and peak at
around one week after the last use of the drug.
The user feels they have to have it and spends
much of their life seeking, buying and using it. They cannot stop
even when other important parts of their life (family, school,
You are most likely to become dependent on
cannabis if you use it every day.
What about skunk and other stronger
The amount of the main psycho-active
ingredient, THC, that you get in herbal cannabis varies hugely from
as low as 1% up to 15%. The newer strains, including skunk,
can have up to 20%. The newer varieties are, on the whole,
two or three times stronger than those that were available 30 years
ago. It works more quickly, and can produce hallucinations with
profound relaxation and elation – along with nervousness, anxiety
attacks, projectile vomiting and a strong desire to eat. They
may be used by some as a substitute for Ecstasy or LSD.
Legally, these strains remain
classified Class B drugs. While there is little research so
far, it is likely that these stronger strains carry a higher risk
of causing mental illness. A major study currently underway,
has already reported problems with concentration and short-term
memory in users of stronger types of cannabis.
Problems with cannabis use
Many – perhaps most – people who use cannabis
do enjoy it. But it can become a problem for some people. A US
defines the problems of cannabis as follows:
“If cannabis controls our lives and our
thinking, and if our desires centre around marijuana - scoring it,
dealing it, and finding ways to stay high so that we lose interest
in all else.”
The website carries the following
questionnaire – which could equally well apply to alcohol use.
"If you answer ‘Yes’ to any of the questions,
you may have a problem.
1. Has smoking pot stopped being fun?
2. Do you ever get high alone?
3. Is it hard for you to imagine a life
4. Do you find that your friends are
determined by your marijuana use?
5. Do you smoke marijuana to avoid dealing
with your problems?
6. Do you smoke pot to cope with your
7. Does your marijuana use let you live in a
privately defined world?
8. Have you ever failed to keep promises you
made about cutting down or controlling your dope smoking?
9. Has marijuana caused problems with
memory, concentration, or motivation?
10. When your stash is nearly empty, do you
feel anxious or worried about how to get more?
11. Do you plan your life around your
12. Have friends or relatives ever complained
that your pot smoking is damaging your relationship with them?”
Reducing cannabis use
Office recently published a guide on how to cut down and stop
cannabis use. It suggests a range of things you can do to
successfully stop using, including:
- drawing up a list of reasons for wanting to change
- planning how you will change
- thinking about coping with withdrawal symptoms
- having a back-up plan.
If you decide to give up cannabis, it may
be no more difficult than giving up cigarettes. You could
- to do it yourself – work through the leaflet on the
Many people will be able to stop on their own. However, if this
- Join a support group, for instance the on-line Marijuana Anonymous
- Talk to your GP or practice nurse. They will have a lot of
experience in helping people to cut down their drinking and to stop
smoking. They can also refer you to more specialist services, such
as a counsellor, support group NHS substance misuse service.
- NHS substance misuse services offer assessment and counselling
for a range of street drugs, aiming to help with
- harm reduction – reducing the impact of the drug on your
- abstinence – stopping completely
- relapse prevention – not starting to use again
- some offer a specific service for cannabis users.
Where can I get more help and
Talk to Frank is an
excellent website. You can order free information leaflets for
different age groups, read real life stories of other people's
experience with drugs and get reliable, factual information.
Helpline: 0800 77 66 00. Use the search facility to get the contact
details of organisations offering practical help and support in
Film Exchange on Alcohol and Drugs
(FEAD): an online resource from leading figures in the alcohol
and drugs field.
Reclassification of cannabis
- Further consideration of the
classification of cannabis under the Misuse of Drugs Act 1971
(2005) Advisory Council on the Misuse of Drugs. Home Office:
- Cannabis use and mental health in young
people: cohort study (2002) George C Patton et al. British Medical
- Cannabis and educational achievement (2003)
Fergusson DM, Horwood LJ & Beautrais AL. Addiction
- Cannabinoids and the human uterus during
pregnancy (2004) Dennedy MC et al.
American Journal of Obstetrics and Gynaecology. 190(1), 2–9.
Cannabis and flying
- Cannabis intoxication and fatal road
crashes in France: population based case control study (2005)
Laumon B et al. British Medical Journal, 331, 1371-1377.
- Marijuana abstinence effects in marijuana
smokers maintained in their home environment (2001) Budney AJ
et al. Archives of General Psychiatry, 58,
- Marijuana use and car crash injury (2005)
Blows S et al. Addiction, 100, 5, 605.
- Self reported cannabis use as a risk factor
for schizophrenia in Swedish conscripts of 1969: historical cohort
study (2002) Zammit S, Allebeck P, Andreasson S, Lundberg I, Lewis
G. British Medical Journal 2002; 325: 1199-1201.
- Cannabis use and psychosis: A longitudinal
population-based study (2002) Van Os J, Bak M, Hanssen M, Bijl RV,
de Graaf R, Verdoux H. American Journal of Epidemiology; 156:
- Cannabis use in adolescence and risk for
adult psychosis: longitudinal prospective study (2002) Arseneault
L, Cannon M, Poulton R, Murray R, Caspi A, Moffit TE. British
Medical Journal; 325: 1212-1213.
- Cannabis use and mental health in young
people: cohort study (2002) Patton GC, Coffey C, Carlin JB,
Degenhardt L, Lynskey M, Hall W. British Medical Journal; 325:
- A longitudinal study of cannabis use and
mental health from adolescence to early adulthood (2000) McGee R,
Williams S, Poulton R, Moffitt T. Addiction; 95: 491-503
- Mental health of teenagers who use cannabis
(2002) Rey JM et al. British Journal of Psychiatry, 180,
- Prospective cohort study of cannabis use,
predisposition for psychosis and psychotic symptoms in young
people. Henquet C et al British Medical Journal, 330, 11-14.
- Tests of causal linkages between cannabis use
and psychotic symptoms (2005) Fergusson DM, Horwood LJ and Ridder
EM Addiction, 100 (3).
- Cannabis-induced psychosis and subsequent
schizophrenia-spectrum disorders: follow-up study of 535 incident
cases (2005) Arendt M et al British Journal of Psychiatry,
187: 510 - 515.
This leaflet was produced by our Public
Education Editorial Board.
Series editor: Dr Philip
Expert review: Dr Eilish
Gilvarry, Dr Zerin Atakan & the Addictions Faculty.
User and Carer input: Members of the
RCPsych Service User Recovery Forum and Carers' Forum.
With grateful thanks to Jane Feinmann.
* November 2012: Please note that this leaflet is
currently under review.
© February 2009 Royal College of
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