Using cannabis can affect memory, concentration and decision-making, according to researchers.
Canadian experts reviewed the scientific evidence surrounding the drug. They warned the effects of cannabis can persist well beyond the period of intoxication. University of Montreal scientists claimed children who smoke the drug may perform worse at school because of its effects.
And adults who use the drug, could also suffer at work or impair their driving ability. The review, published in the journal Addiction, concluded the impairments ‘may be worse in regular and heavy users’.
More prevention measures are needed in schools to prevent adolescents using the drug chronically, researchers said.
Montreal academics analysed existing cannabis studies involving more than 43,000 people.
All of the study papers looked at how the drug affected memory, learning, attention, processing speed, language and motor function.
Researchers looked at the extent to which cannabis affects cognitive performance immediately after taking it and in the longer term.
The studies analysed ranged from experiments involving as few as 65 volunteers, to ones including thousands.
And they involved a range of smoking habits — from daily use to abstinence.
Heavy and frequent use was found to hamper decision-making, flexible thinking and self-control.
Other studies suggested regular cannabis use hampered the ability of people being to control their behaviour, and left them less able to make ‘appropriate decisions’.
Research also showed adults given THC — the psychoactive substance that gives off a ‘high’ — performed worse in memory tests.
But not all of the studies agreed, with some of the analyses uncovering no effect on memory caused by regular cannabis use.
Dr Alexandre Dumais, a psychiatrist and co-author of the study, argued cannabis can impair ‘several areas of cognition’.
These include ‘problems concentrating and difficulties remembering and learning, which may have considerable impact on users’ daily lives’.
‘Cannabis use in youth may consequently lead to reduced educational attainment, and, in adults, to poor work performance and dangerous driving,’ he added.
‘These consequences may be worse in regular and heavy users.’
Previous studies have shown the drug — which is used medicinally across the world — can alleviate depression, anxiety and stress.
But heavy use may worsen depression in the long term by reducing the brain’s ability to let go of bad memories.
It can also worsen mental health problems among people who already have them, or increase users’ risk of psychosis or schizophrenia, according to research.
Marijuana can be prescribed for medical uses in more than half of US states, where it is used to combat anxiety, aggression and sleeping problems. Researchers are also looking into whether it could help people with autism, eczema or psoriasis.