Daily use of cannabis in teenagers under 17 triggers suicide risk and teenagers who use pot daily are 60 percent less likely to complete high school or get a degree than peers who have never taken the drug, researchers said on Wednesday.
They are also nearly seven times likelier to attempt suicide and are almost eight times likelier to use other illicit drugs later in life.
The data, published in the journal The Lancet Psychiatry, comes from an analysis of three large, long-running studies in Australia and New Zealand.
"Our findings are particularly timely, given that several US states and countries in Latin America have made moves to decriminalise or legalise cannabis, raising the possibility that the drug might become more accessible to young people," said Richard Mattick, a professor at the National Drug and Alcohol Research Centre in New South Wales.
The three studies considered the welfare of several thousand young people, who were assessed regularly between the ages of 13 and 30.
The analysis divided them into five categories of cannabis use frequency over 12 months in "mid-adolescence" -- meaning, over a period before the individual turned 17. The categories ranged from never to daily use.
And it compared this to their educational achievements.
As far as possible, the review took into account factors like gender and socio-economic status that could skew the picture.
"The estimates... suggested that individuals who were daily users before age 17 years had odds of high-school completion and degree attainment that were 63 percent and 62 percent lower, respectively, than those who had never used cannabis," the study reported.
The researchers conceded it was possible that some cannabis users turned to the drug after dropping out of school, rather than before.
But the figures clearly showed that the more cannabis was consumed, the likelier it was that a teen would not finish secondary education.
The link with suicide risk was not a surprise, given previous research evidence of an association between suicidal thoughts and heavy cannabis use, the authors said.
But the study swept aside the notion that teenaged cannabis smokers were more prone to depression or to be dependent on welfare.
No such association was found in the data, they said.
Other experts said the results of the study were unsurprising but still provided needed data to support anecdotal evidence about pot use.
"None of the findings will surprise mental health workers, and indeed previous studies have reported similar findings for each of the outcomes separately," said Robin Murray, a professor at King's College London's Institute of Psychiatry, Psychology and Neuroscience.
But, Murray added in comments reported by Britain's Science Media Centre, "This study reminds us that it is important to discourage cannabis use among teenagers."
"Educational campaigns outlining the risks of heavy cannabis use are warranted whatever the legal status of cannabis."
He also regretted that the probe did not explore the impact from high-strength cannabis, which now constitutes a major component of sales of the drug.
A contrasting view came from Professor David Nutt, former president of the European College of Neuropsychopharmacology, who argued that cannabis abuse was rooted in the same problems as alcohol abuse.
"In all cases it is likely that significant proportions of the users have pre-existing problems and seek cannabis as a way out," he said.
"These data don't really help with the legalisation debate as it's been legal in Holland for decades with little impact," Nutt added. Indeed, young people may use less once it's legal, if they were rebelling by using it.?