Cannabis and mental health
UNDER REVIEW OCTOBER 2017
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 - cannabis.
Cannabis is the most widely used substance in the UK. Even
though there has been a steady reduction of use since 1996, about
2.3 million 16-59 year-olds have reported using cannabis in the
past year. Frequent use of cannabis is about twice as
likely amongst young people, and nearly 5.3 million 16-24
year-olds have used it in the last year.
In spite of government and media warnings about health risks,
many people see cannabis 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, research over the last 10 years has suggested
that it can have serious consequences for people, such as the
development of an enduring psychotic illness, particularly in those
who are genetically vulnerable.
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 the main active ingredient, namely THC
(tetrahydrocannabinol). 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, Knock Out or Destroyer.
Over the last 15 years, skunk has invaded the
street market and its THC content is about 2-3 times higher than
the 'traditional' cannabis used in earlier years. In the UK, most
sold materials is home grown because of a loop hole in the
law making it legal to buy seeds over the internet.
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. 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 (CBD),
delta-8-tetrahydrocannabinol and cannabinol. Apart from CBD,
these compounds are psychoactive, the strongest one being
delta-9-tetrahydrocannabinol. The stronger varieties of the
plant contain little CBD, whilst the delta-9-THC content is a lot
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 THC-like substances produced
naturally by the brain itself – these are called endocannabinoids.
Even though chemically THC is not similar to the natural
endocannabinoids, it can fit, like a key, into the same receptor
lock and interferes with the normal functioning of the
Most of these receptors are found in the parts
of the brain that influence emotion, 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.
Even though THC can produce relaxation, if
higher amounts are consumed, it can have the opposite effect by
increasing anxiety. Some cannabis users may have
unpleasant experiences, including confusion, hallucinations,
anxiety and paranoia, depending on their mood and
Some users may experience psychotic symptoms with hallucinations
and delusions lasting a few hours, which can be very unpleasant.
Even though these unpleasant effects do not last long, since
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 and reduce
motivation. Some researchers also suggest that long-term use can
lead to irreversible, but minor cognitive deficits.
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. 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 so".
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.
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
However, does cannabis cause depression and schizophrenia or do
people with these disorders use it as a medication?
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.
- Psychoses - schizophrenia and bipolar
There is now sufficient evidence to show that
those who use cannabis particularly at a younger age, such
as around the age of 15, have a higher than average risk of
developing a psychotic illness, such as schizophrenia or bipolar disorder.
These studies also show that the risk is dose-related. In other
words, the more cannabis someone used, the more likely they were to
develop a psychotic illness. Furthermore, a study in Australia
recently showed that those who used cannabis could develop the
illness about 2.70 years earlier than those who did not.
Why should teenagers be particularly
vulnerable to the use of cannabis? It is thought that this has
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
It is also known that not everyone who uses
cannabis, even at a young age, develops a psychotic
illness. The available research shows that those who have a
family history of a psychotic illness, or those who have certain
characteristics such as schizotypal personality,
or possibly have certain types of genes, may increase the risk of
developing a psychotic illness following the regular use of strong
Physical health problems
Even though the main risk to physical health from cannabis is
probably from the tobacco that it is often smoked with, new
research has found that the cannabis plant also contains
cancerogenic mutagens that can affect people's lungs.
Is there such a thing as ‘cannabis
Some people may develop temporary psychotic
symptoms, such as hallucinations and delusions, which resolve
themselves within hours or a few days without any help. People who
experience these temporary effects do not normally come to the
attention of psychiatric services.
Previously, if a person was known to be a heavy cannabis
user, and came to psychiatric services with a psychotic
condition, the term "cannabis psychosis" was used. Often
these problems turned out to be long-lasting, and the person
developed enduring severe mental illness. However, we have
now found that these cases should be diagnosed according to
person's symptoms, including their use of cannabis which could be a
possible trigger for psychosis.
It may be this group of people are particularly vulnerable to
the effects of cannabis, and so should probably avoid it in the
Is cannabis addictive?
Yes. Even though in the past cannabis was not
thought to be addictive, current evidence now suggests that it can
be, particularly if used regularly. Cannabis has the features
of addictive drugs such as the development of:
- tolerance – which means having to take more
and more to get the same effect. In heavy users, you can experience
withdrawal symptoms such as:
- decreased appetite
- sleep difficulty
- weight loss
- aggression and/or increased irritability
- 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.
Even though low THC content in herbal
cannabis can reduce anxiety, higher amounts of THC produces
anxiety. They may be used by some as a substitute for Ecstasy or
Research has also shown that skunk has taken over the street
market in the UK, and the low THC varieties are less available.
Most skunk is home-grown, and many people, including young
teenagers, are introduced to large amounts of THC, even at first
Legally, these strains remain
classified Class B drugs. Research shows that there is a
higher risk of developing a psychotic illness with cannabis
with high levels of THC, and if you are a regular
user. The easy availability of skunk carries a specific risk to
young people with a genetic predisposition. Cannabis with high
amounts of THC can also cause cognitive problems, such as problems
with short-term memory and processing speed.
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.
- Has smoking pot stopped being fun?
- Do you ever get high alone?
- Is it hard for you to imagine a life without
- Do you find that your friends are determined
by your marijuana use?
- Do you smoke marijuana to avoid dealing with
- Do you smoke pot to cope with your
- Does your marijuana use let you live in a
privately defined world?
- Have you ever failed to keep promises you
made about cutting down or controlling your dope smoking?
- Has marijuana caused problems with memory,
concentration, or motivation?
- When your stash is nearly empty, do you feel
anxious or worried about how to get more?
- Do you plan your life around your marijuana
- Have friends or relatives ever complained
that your pot smoking is damaging your relationship with
Reducing cannabis use
Office 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, you
- to do it yourself – work through the leaflet on the FRANK website
Many people will be able to stop on their own. However, if you
- 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 a
website with a lot of useful information. 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: 0300 123 6600.
Thet also have a chat line.
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.
Crime Survey for England and Wales 2011/2012
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.
- The association between cannabis use and earlier age at onset
of schizophrenia and other psychoses: meta-analysis of possible
confounding factors. Myles N, Newall H, Nielssen O, Large M. Curr.
Pharm Des. (2012);18(32):5055-69.
This leaflet was produced by our Public
Education Editorial Board.
Series editor: Dr Philip
Expert review: Dr Zerin
User and Carer input: Members of the
RCPsych Addictions Faculty Patients' and Carers' Liaison
With grateful thanks to Jane Feinmann.
This leaflet reflects the most up-to-date
evidence at the time of writing.
© Royal College of Psychiatrists.
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