The Good, the Bud and the Ugly: What 20 Years of Research Teaches Us About Cannabis

Cliff DesPeaux/Reuters

On Monday, the scientific journal Addiction released a sweeping new review of 20 years of research into the recreational use of marijuana. Dr. Wayne Hall, director of the Centre for Youth Substance Abuse Research at the University of Queensland and an adviser to the World Health Organization, began the review in 1993. At that time, there were "very few epidemiological studies of the health effects of cannabis," he says. The existing scientific literature was "dominated" by "animal studies from the 1970s," and the only human laboratory studies available were from the late '70s and early '80s. And even those analyzed only "the effects of sustained cannabis use over 7–35 days on the health of college students."

What changed to make a review like this possible? Two things: First, people are smoking more weed. "During the past half-century," the authors say, "recreational cannabis use has become almost as common as tobacco use among adolescents and young adults," which means there are simply more weed smokers to study.

Secondly, today's weed is about four times as strong as it was 30 years ago. "THC content of cannabis has increased in the United States from <2% in 1980 to 8.5% in 2006," the review found.

The purpose of this review, then, was to examine "changes in the evidence on the adverse health effects of cannabis since 1993." And it turns out that, overall, what we thought about cannabis in 1993 was pretty accurate. Or in scientific terms: "The epidemiological evidence has strengthened for many of the probable adverse health effects that we identified in 1993."

First, though, some good news. The review found the research still shows you can't overdose on cannabis. "In 1993," the review says, "the evidence indicated that the risk of a fatal overdose from using cannabis was extremely small." That's still true. The amount of marijuana required to trigger an overdose is somewhere between 15 and 70 grams, which is "a far greater amount of cannabis that even a very heavy cannabis user could use in a day," the review found.

Most of the news is bad, though. First, don't smoke pot while driving. In 1993, scientists knew that cannabis produced "impairments in reaction-time, information-processing, perceptual-motor coordination, motor performance, attention and tracking behaviour," which "suggested that cannabis could potentially cause car crashes if users drove while intoxicated, but it was unclear whether in fact cannabis use did so." Today, scientists are sure. A meta-analysis "found that recent cannabis use (indicated by THC in blood or self-reported cannabis use) doubled the risk of a car crash," the review found.

Second, cannabis addiction is real, and you're more likely to become dependent if you start smoking in adolescence. One in six people who start using cannabis regularly in adolescence develops a dependency, compared with around one in 10 who start using it as adults.

Finally, smoking too much weed might actually make you dumb—but only if you smoke like a chimney. Increasingly sophisticated studies since 1993 "have consistently found deficits in verbal learning, memory and attention in regular cannabis users," the review found, but noted that "these effects on IQ were found only in the small proportion of cannabis users who initiated in adolescence and persisted in daily use throughout their 20s and into their 30s."

"It is now difficult to argue that cannabis dependence does not require professional attention," Hall said.