Psychological and Physical Effects


What are the psychological effects of cannabis?

Cannabis produces its effects on the brain through interactions between its psychoactive components (mainly THC) and particular receptors found in our brain cells. It is thought that these receptors usually interact with naturally occurring brain chemicals, one of which has been identified and called anandamide (ananda means ‘bliss’ in Sanskrit). There is little research on this substance, but it has pain-relieving and tranquillising effects in animals.

Cannabis also causes dose-related changes in brain chemicals such as dopamine and serotonin, which are involved in the control of mood and emotion. Different strains of cannabis have different proportions of THC and other chemical (cannabinoid) components, and these accounts for some of the variation in effects between different people, or different occasions.

The main effect of cannabis on mood is that it produces euphoria – a high. Most people’s reason for taking it is ‘pleasure’. The high may be accompanied by changes in perception: colours may seem brighter, music more vivid, emotions more poignant and meaningful. However, it can also cause severe anxiety and panic, and psychosis. Paranoia and hallucinations may occur with high doses.

It is suggested that the unwanted psychological effects of cannabis can be classified as:

  • short-lived effects such as anxiety, panic, depression and psychosis, which usual occur after excessive consumption of the drug
  • effects on pre-existing mental illness
  • cannabis as a risk factor for mental illness
  • dependency and withdrawal.

Cannabis, psychosis and schizophrenia

Depending on the kind that has been used, and your susceptibility to it, cannabis can cause psychotic experiences such as hallucinations, fantasies, depersonalisation and derealisation (feeling out of touch with yourself or your surroundings), feeling a loss of control, fear of dying, irrational panic and paranoid ideas.

In theory, cannabis may cause a psychotic reaction in the following ways:

  • Taking a high dose may cause a psychotic reaction with hallucinations or confusion, which goes away after the drug is stopped.
  • It may lead to a long-term psychosis that does not go away when the drug is stopped.
  • Long-term use may induce psychosis that gets a bit better if the drug is stopped.
  • Cannabis may be a trigger for serious mental illness, such as schizophrenia. 

Among people who already have a diagnosis of psychotic illness, cannabis users are thought to experience more psychotic relapses. Again, experience depends on the strength of the type used as well as frequency of use.

Research in young people suggests that using cannabis as a teenager increases the likelihood of experiencing symptoms of schizophrenia in adulthood, and early cannabis use (by age 15) confers greater risk than using it later on (by age 18). This research suggests that, although the majority of adolescents are not harmed by using cannabis, a small minority are.

Results of a Swedish study suggest that cannabis increases the risk of schizophrenia by 30 per cent. However, this does not appear to be reflected in the figures for schizophrenia in the population in general, which have remained constant over a long period. This study also concludes that cannabis has few harmful effects overall, but that there is a potentially serious risk to the mental health of people who use cannabis, particularly in the presence of other risk factors for schizophrenia.

Researchers who examined further published evidence on cannabis and psychosis in 2004 came to the conclusion that, for any individual, using cannabis doubles the risk of developing schizophrenia in later life, and, for the population as a whole, elimination of cannabis use would reduce the incidence of schizophrenia by about 8 per cent, if you assume that it has a causal effect. Cannabis use alone does not cause psychosis, but it is one of the things that may contribute to its development; therefore, using cannabis increases the risk, and some cases of psychosis could be prevented by discouraging cannabis use among vulnerable young people. A response to this report further suggested that adolescents may be more vulnerable to the adverse effects of cannabis than are adults because their brains are still developing. A further study concluded that cannabis use increases the risk of psychotic symptoms in young people, but has a much stronger effect in those with evidence of a predisposition for psychosis (such as a family history of mental illness).

Support for this suggestion is provided by a study in young people of a particular enzyme called COMT (catechol-O-methyl transferase), which is involved in the regulation of the brain chemical dopamine (psychosis is thought to be associated with high levels of dopamine). COMT is produced by a gene, two variants of which occur in the population, and produce two forms of the enzyme, which break down dopamine to different degrees. As with all genes, people inherit one copy from each parent. The study found that people who inherited two ‘good’ subtypes could use cannabis without it affecting their mental health; those with one of each subtype were at slightly more risk of psychosis with cannabis; while in those who had two ‘bad’ subtypes the risk was increased ten-fold.

A further study, reported in The Lancet in July 2007, which analysed reports from 35 published studies, found ‘a consistent increase in incidence of psychosis outcomes in people who had used cannabis’. Their analysis suggested that there was a 40 per cent increase in risk of psychosis in study participants who had ever used cannabis. Effects were larger with frequent use, with an increased risk of up to 200 per cent in those who used cannabis most heavily.

In a written answer to the House of Commons on 18 July 2005, Rosie Winterton MP said that admissions associated with cannabis use rose to 710 in 2003-2004, from 580 in the previous two years. There is some evidence that, since cannabis was down-graded in the UK from a class B to a class C drug in January 2004, psychiatrists have seen further increasing numbers of people being hospitalised with psychotic episodes associated with cannabis use. This is in spite of the fact that overall numbers of people using cannabis have fallen, and a further indication that of those people who use cannabis, more of them are using strong varieties which are more likely to cause adverse effects on mental health.


Other psychological effects

There are other, less frightening, effects of cannabis on mental functioning. Heavy users may experience lethargy, loss of communication skills and a general lack of ambition. Their faculties do return, however, if they stop smoking. The idea that cannabis use causes long-term memory loss may arise because most heavy smokers are too ‘stoned’ to absorb much information in the first place: cannabis is not thought to affect memories which are already in place, only the ability to store new ones.

Cannabis impairs mental flexibility and problem-solving abilities. It also decreases attention and concentration, and impairs judgement and complex co-ordination (for example, the ability to drive a car or operate machinery). Unlike many drugs, including alcohol, it does not normally make people aggressive; however, there is some evidence that people may commit acts of violence under the influence of cannabis-induced psychotic delusions.

In a study of adolescents in Australia, those who used more cannabis were more likely to develop depression and anxiety, and this was more marked in girls. Young women who used cannabis daily were five times more likely to have depression and anxiety than non-users, and using cannabis weekly doubled the rates of anxiety and depression. There was no evidence that girls who were already depressed before using cannabis were more likely than others to use it.

Other studies, using self-reported questionnaires with groups of users, have reported panic attacks and anxiety in a significant number of users, and depression, tiredness and low motivation.

Recent research using a rat model of depression suggests that cannabis may act as an antidepressant at low doses, increasing serotonin levels in the brain. Higher doses reverse this effect, causing serotonin levels to fall below those of the control group. The researchers reported that this effect was mirrored in their patients who were regular users of cannabis.

What are the physical effects of cannabis?

Cannabis smokers experience the same health problems as tobacco smokers; including bronchitis, emphysema (a lung condition causing breathing problems), bronchial asthma, and lung cancer. Immediate effects may include light-headedness and faintness. Other effects include increased heart rate, dry mouth, red eyes, impaired motor co-ordination, hunger, and an increased desire for sugar. It can reduce sperm production, disrupt the menstrual cycle, and affect the growth of an unborn child. Extensive use increases the risk to the lungs and reproductive system, and suppresses the immune system.

It is unlikely that anyone has ever died from an overdose of cannabis, but people with heart disease or high blood pressure may be at risk, because it increases the heart rate and places greater stress on the heart.

Cannabis has effects similar to alcohol and benzodiazepines (minor tranquillisers) on the performance of tasks, including both thinking and motor co-ordination. It impairs concentration and short-term memory, and slows reaction times. It will therefore reduce the ability to perform skilled tasks such as driving. After alcohol, cannabis is the drug most commonly found in drivers following fatal accidents, and a French study has shown that driving under the influence of cannabis increases the risk of road accidents.

View as PDF


The Have I Got A Problem website is a free online resource to help people better understand any issues or concerns they may have about mental health or addiction. The website includes resources specifically focused to; general Mental Health, Depression, Stress, Anxiety, Insecurities, Self-harm Schizophrenia, Bipolar, Anger Management, Eating Disorders, Coping, general Addiction, Alcohol, Smoking, Gambling, Drugs, Cocaine, Heroin, Marijuana (Cannabis) Ecstasy, PCP, Mephedrone, Ketamine & Crystal Meth.

The site was created to give the public information to help them understand mental health and addiction issues and to assist people in making better informed decisions about their life and personal choices. was created and is run by 'Advising Communities’, which is a UK registered charity (Charity No. 1061055)


"I don't miss the fear that arose when I'm driving my car every time a police car hovers into view."


MoreSend us your Quotes

Tips & Hints

  • Decide to make a change

    For many people struggling with addiction, the biggest and toughest step toward recovery is deciding to make a change. It’s normal to...
  • Keep triggers and cravings in check

    While getting sober from drugs is an important first step, it’s only the beginning of the recovery process. Once sober, the brain nee...
  • Don’t let relapse keep you down

    Relapse is a common part of the recovery process from drug addiction. While relapse is understandably frustrating and discouraging, it ...
  • Make a life change

    The first step to fighting addiction is to make a decision that it is time to make a life change. That life change will reflect your de...
  • No more secrets

    It’s time to clean out the closet and take inventory. No hiding because of shame. No hiding because of fear. You are not alone in you...
  • More Tips & Hints