Let's set aside, for the moment, all the debate over whether marijuana is a gateway drug whose users may be propelled toward harder, more dangerous drugs.
There's a new, more sinister concern about cannabis. According to some scientists, it may be directly linked to mental illness, including schizophrenia, in young pot smokers.
CBC's The Nature of Things takes an unsettling look at the new evidence tonight in The Downside of High (8 p.m., CBC), an hour-long documentary written and directed by Bruce Mohun and narrated by series host David Suzuki.
The Downside of High is a particularly effective examination of its subject because it straddles the line between cold, hard scientific information and up-close human experience. As an entry point to the discussion, the film's makers introduce us to three young British Columbians whose lives were sent careening sideways after they started experimenting with pot.
Each first tried smoking marijuana in the usual peer-group environment; each quickly got hooked on getting high; each soon developed deeply delusional behaviour -- hearing voices, extreme paranoia, fear and panic -- that ultimately landed them in hospital psychiatric wards for extended stays.
And each, along with the doctors who have helped them in the slow effort to rebuild their lives, is convinced that their mental illnesses were triggered by marijuana use.
That's where the scientists come in.
The Downside of High examines the work of several researchers who have studied the link between pot and schizophrenia, beginning with a groundbreaking 1987 Swedish study that followed 50,000 young army recruits for more than 15 years and concluded that those who used marijuana during their teen years were six times more likely to be diagnosed with schizophrenia during the next decade and a half of their lives.
Dutch researcher Dr. Jim Van Os included this study as he prepared a comprehensive overview of all the available data on the topic; his admittedly more conservative conclusion is still cause for concern.
"We found that cannabis use nearly doubles the risk of developing future psychotic states," he explains, "be it isolated psychotic symptoms or clinical psychotic disorders, like schizophrenia."
Van Os's research also concluded that teens who begin using marijuana before the age of 15 may be four times as likely to develop schizophrenia.
Part of the problem, according to The Downside of High, is the fact pot growers -- including the "B.C. Bud" purveyors who call Canada's West Coast home -- continue to develop new breeds of weed that are exponentially more potent than the "harmless" pot that fuelled 1960s and '70s counterculture.
In addition to containing much higher levels of THC (tetrahydrocannabinol), marijuana's active (and sometimes psychosis-producing) agent, new strains of pot also contain much less CBD (cannabidiol), which is thought to protect pot users against the drug's psychosis-inducing properties.
Van Os and other scientists have also found evidence that there's a genetic link that makes some people much more likely to suffer marijuana-induced mental illness; some of the most current research is aimed at developing an accurate test that might allow parents to learn whether their teenagers are part of the high-risk group when it comes to pot and mental-health problems.
As one might expect in a film focused on scientific research, nothing here is completely conclusive. Not all scientists agree on the cause-effect relationship; not all pot users are at risk of encountering mental illness. But there certainly is much to consider in The Downside of High, especially for parents and teens who find themselves standing at a much different gateway than they ever thought the issue of recreational drug use might create.