Does cannabis make young people more vulnerable to psychotic illnesses? We toke on the evidence.

Certain scientific research points to a link between cannabis use and mental illness, although experts are divided over the exact role the drug plays in affecting your mental health.

The four main theories

  • The first theory suggests that cannabis use can cause mental disorders, or worsen existing psychotic symptoms (paranoia, hallucinations and delusions) among people with schizophrenia and bipolar affective disorder;
  • The second argues that cannabis use can trigger psychotic symptoms in those who are already prone to mental disorders - for instance, those who have close family members with illnesses such as schizophrenia;
  • The third states that cannabis use doesn't cause or trigger mental illness, but that some people are likely to experience both simply as a result of common circumstances, such as stress and anxiety caused by family difficulties, unemployment or other drug use;
  • The fourth claims that any link is down to the fact that many people with mental health disorders use cannabis to ease the symptoms they're experiencing.

Are young people more at risk?

There's increasing evidence to suggest that long-term use of cannabis can lead to the development of psychosis, particularly among those who start using it in their teens. A study in New Zealand found that those who had used cannabis three times or more before the age of 15 were much more likely to experience symptoms of schizophrenia by the time they were 26. The research team concluded that there's a vulnerable minority of teenagers for whom cannabis is harmful, but added that, "We're not saying that cannabis is the major cause of schizophrenia, but it's a risk factor."

A study of 2,500 14 to 24 year-olds in Munich, Germany, found that among those who had no vulnerability to psychosis, cannabis use moderately raised the risk of developing signs of psychosis later on, from 16% to 25%. But for those who were considered vulnerable to psychosis (who had shown psychotic symptoms by the age of 11), scientists found a stronger link. For these people, cannabis use almost doubled their chance of developing psychosis, from 25% to 50%. Also, the risks increased with the more cannabis they smoked, and the earlier they smoked it.

Meanwhile, a team from Bristol and Cardiff Universities published a paper that claims cannabis users are 40% more likely to suffer a psychotic illness than non users. The research suggests that although the risk of getting a psychotic illness like schizophrenia remained low, it could be a factor in 14% of psychotic problems in young adults in the UK. The researchers looked at 35 studies on cannabis and its link to mental health.

In your genes?

Further studies have revealed that one in four people carry genes that increase vulnerability to psychotic illness if they smoke cannabis as a teenager. If you inherit types of a gene called catechol-O-methyl transferase (COMT), it's thought that cannabis is around five times more likely to trigger schizophrenia and other similar disorders.

COMT is involved with the breaking down of a key brain chemical called dopamine, and comes in two forms - one inherited from each of your parents. The 'bad' version of the gene lessens your brain's ability to break down dopamine. People with psychosis have increased dopamine in one area of the brain, which could be linked to the presence of a 'bad' COMT gene. According to the research:

  • If you have two copies of the 'normal' version, smoking cannabis has little effect on your mental health. In this instance, you have a 3% risk of developing psychotic illness whether you smoke cannabis or not;
  • If you have one 'normal' and one 'bad' version, smoking cannabis slightly increases the risk of psychosis;
  • If you have two 'bad' versions, smoking cannabis as a teenager increases the likelihood of developing psychosis by 15%.

What's wrong with the research?

Critics of the recent findings argue that it's often very difficult to account for other mental health issues that may contribute to the development of psychosis. Also, researchers in Australia have found that, despite a big rise in cannabis use over the past 30 years, the proportion of people with schizophrenia has remained roughly the same. Opponents point out that if cannabis really was causing mental illness in certain people, there should be a growing number of people with disorders like schizophrenia to match the rise in the number of teenagers smoking cannabis. Another criticism of the research is that it does not address the risk of more concentrated forms of cannabis, such as skunk, which are now widely available. 

It's hardly news, but...

Smoking harms your health. So, even if you're not genetically predisposed to developing psychotic illnesses, you'd be daft to forget that smoking joints can harm your cognitive functions, as well as increase the risk of serious respiratory illnesses, including lung cancer.


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