Daily consumption of marijuana may increase an adolescent's risk of having recurrent psychotic- like experiences by 159 per cent, a study warns.
Psychotic-like experiences include perceptual aberration, ideas with unusual content and feelings of persecution, researchers said.
"Although they may be infrequent and thus not problematic for the adolescent, when these experiences are reported continuously, year after year, there is an increased risk of a first psychotic episode or another psychiatric condition," said Josiane Bourque, a doctoral student at Universite de Montreal (UdeM) in Canada.
"Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms," Bourque said.
Bourque and her supervisor, Patricia Conrod at Sainte Justine University Hospital Research Centre hypothesised that impairments in cognitive development due to marijuana misuse might in turn exacerbate psychotic-like experiences.
This hypothesis was only partially confirmed, however. Among the different cognitive abilities evaluated, the development of inhibitory control was the only cognitive function negatively affected by an increase in marijuana use.
Inhibitory control is the capacity to withhold or inhibit automatic behaviours in favour of a more contextually appropriate behaviour.
Conrod's team has shown that this specific cognitive function is associated with risk for other forms of substance abuse and addiction.
"Our results show that while marijuana use is associated with a number of cognitive and mental health symptoms, only 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," Bourque said.
These findings have important clinical implications for prevention programs in youth who report having persistent psychotic-like experiences, researchers said.
"While preventing adolescent marijuana use should be the aim of all drug strategies, targeted prevention approaches are particularly needed to delay and prevent marijuana use in young people at risk of psychosis," said Patricia Conrod, professor at UdeM.
The study's results are based on a cohort of about 4,000 adolescents aged 13 years old from 31 high schools in the Greater Montreal area in Canada.
These teens are followed annually from Grade 7 to Grade 11. Every year they fill out computerised questionnaires to assess substance use and psychiatric symptoms.
The teens also complete cognitive tasks to allow the researchers to evaluate their IQ, working memory and long-term memory as well as their inhibitory control skills.
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