From occasional use of marijuana to indulging every day increases the risk of psychosis by up to 159%

• Marijuana use significantly reduces your ability to resist inappropriate behavior
• The drug is thought to cause psychosis by increasing a user's depression risk
• A link has previously been established between depression and psychosis' onset
• Psychosis-like experiences may be a serious concern if they occur repeatedly
• Symptoms include feeling something external is part of you and persecution 

By Alexandra Thompson 

Going from being an occasional marijuana user to indulging every day increases the risk of psychosis by up to 159 percent, new research reveals.

Marijuana is thought to cause psychosis-like experiences by increasing a user's risk of depression, a study found. The two mental health conditions have previously been linked.

Frequently abusing the substance also significantly reduces a user's ability to resist socially unacceptable behavior when provoked, the research adds. 

Study author Josiane Bourque from the University of Montreal, said: 'Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms.

'[Psychosis symptoms] may be infrequent and thus not problematic for the adolescent, when these experiences are reported continuously, year after year, then there's an increased risk of a first psychotic episode or another psychiatric condition.'

How the study was carried out  

Researchers from the University of Montreal analyzed around 4,000 13-year-olds from 31 high schools in the surrounding area.

Every year for four years, the study's participants completed questionnaires about any substance abuse and psychotic experiences.

Psychotic symptoms included perceptual aberration - for example feeling that something external is part of their body - and thinking that they have been unjustly badly treated.

The participants also completed cognitive tasks that allowed the researchers to assess their IQ, memory and stimuli response.

Key findings

Results revealed that going from being an occasional marijuana user to abusing the substance once a week or as often as every day, increases the risk of psychosis-like experiences by 159 percent.

The results further demonstrated that marijuana use reduces a person's ability to resist socially unacceptable behavior in response to a particular stimulus.

An increased risk of depression as a result of frequent marijuana use is thought to be behind psychosis' onset. The two mental health conditions have previously been linked. 

The findings were published in the Journal of Child Psychology and Psychiatry.

What the researchers say

Ms Bourque said: 'Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms.

'[Psychosis symptoms] may be infrequent and thus not problematic for the adolescent, when these experiences are reported continuously, year after year, then there's an increased risk of a first psychotic episode or another psychiatric condition.

'An increase in symptoms of depression – such as negative thoughts and low mood – could explain the relationship between marijuana use and increasing psychotic-like experiences in youth.'