Teenagers who use cannabis daily run a higher risk of becoming drug dependent, committing suicide or trying other drugs and are less likely to succeed at their studies than those who avoid it, researchers report.
In an analysis of studies on cannabis, the scientists said these long-term health and life effects were important since several countries are planning to relax legislation on it.
Cannabis is the most widely used illicit drug worldwide, despite a trend towards decriminalising it in some countries.
"Our findings are particularly timely, given that several U.S. states and countries in Latin America have made moves to decriminalise or legalise cannabis, raising the possibility the drug might become more accessible to young people," said Richard Mattick, a professor at Australia's National Drug and Alcohol Research Centre at the University of New South Wales, who co-led the study.
Cannabis smoking findings
Using data from three large and long-running studies, the researchers found that people who smoke cannabis daily before the age of 17 are more than 60% less likely to complete high school or obtain a university degree.
Read: Crack down on cannabis, world body tells US
The meta-analysis also indicated that daily users of cannabis during adolescence are seven times more likely to attempt suicide, have an 18 times greater chance of cannabis dependence, and are eight times as likely to use other illicit drugs in later life.
"Policymakers need to be aware that early use of cannabis is associated with a range of negative outcomes for young adults that affect their health, wellbeing, and also their achievements," said Edmund Silins, also of the National Drug and Alcohol Research Centre, who presented the findings in a media teleconference.
Clear and consistent link
Recent data show that young people in some countries are starting to use cannabis at a younger age than before and that more adolescents are using cannabis heavily.
Read: Booze and pot bad for teens in various ways
In the United States, some 7% of high-school seniors are daily or near-daily cannabis users, while in England, 4% of 11 to 15-year-olds report cannabis use during the past month.
In Australia, around 1% of 14- to 19-year-olds are daily users of the drug, while 4% use it weekly.
Legislation of cannabis
Silins said any changes to cannabis legislation should be carefully assessed to ensure they will help reduce adolescent cannabis use and prevent its potentially adverse effects.
The study, published in The Lancet Psychiatry journal, analysed data on up to 3765 participating cannabis users in terms of seven developmental outcomes up to the age of 30 years.
Those factors were completing high school, obtaining a university degree, cannabis dependence, use of other illicit drugs, suicide attempts, depression and welfare dependence.
It found clear and consistent associations between frequency of cannabis use during adolescence and most young adult outcomes investigated, even after controlling for potential confounding factors including age, sex, ethnicity, socioeconomic status, use of other drugs and mental illness.
Cannabis consumption increasing
Importantly, they also noted that risks increased relative to dose, with daily users showing the strongest effects.
Experts not involved in the research said its findings were particularly worrying given trends suggesting rising cannabis consumption.
Read: Legalising marijuana cuts drug overdose deaths
"This new study ties in well with previous research into the mental health effects of heavy cannabis smoking during adolescence, a period where the mind and brain are still developing," said Michael Bloomfield, a psychiatrist at University College London.
Robin Murray, a professor of psychiatric Research, King's College London's Institute of Psychiatry said the study "reminds us that it is important to discourage cannabis use among teenagers, and that educational campaigns outlining the risks of heavy cannabis use are warranted whatever (its) legal status."
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Image: Teenage boy smoking from Shutterstock
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