Students who only smoke marijuana do better at school than classmates who smoke just tobacco, or who smoke both tobacco and pot, says a new study, which tracked substance use among teens over 30 years.

Researchers from the University of Toronto’s Dalla Lana School of Public Health analyzed data from a survey administered to nearly 39,000 Ontario students between 1981 and 2011. The Centre for Addiction and Mental Health asked students in Grades 7, 9 and 11 about their tobacco and marijuana use, and their academic performance.

The study found that marijuana-only users did better at school than their counterparts who smoked only cigarettes or who smoked both cigarettes and marijuana. However, the findings reflect the fact that fewer students smoke tobacco today compared to 30 years ago, and those that do make up a very “marginalized, vulnerable” population, says lead study author Michael Chaiton, assistant professor in epidemiology and public health policy.

About 92 per cent of tobacco users also use marijuana, the study found. However, only 25 per cent of marijuana uses also smoke tobacco.

“It’s better relatively,” Chaiton says of marijuana-only users’ academic performance.

Marijuana users don’t outperform non-users, Chaiton says.

“Now there is a distinction between marijuana use and co-use with other substances, and it’s an indication of the changing social norms. So it’s not an absolute that they do better; it’s that social norms have changed and the population of people who use marijuana are more like the general population.”

The study was published in the March edition of the Journal of School Health.

In the 1980s, when the study began, there was less marijuana use among students. And those who did smoke pot also smoked tobacco. At the time, pot use among tobacco smokers was very low.

Thirty years later, that had switched, the researchers found. As tobacco use declined, marijuana use shot up. And among the remaining tobacco users, marijuana use is now very high.

One reason for the statistical switch, Chaiton says, is the effectiveness of anti-tobacco messaging in recent years.

“The population of youth smokers right now is one that is a fairly marginalized population, quite a vulnerable population, so they are at high rates of cannabis use but also of other drugs and other behaviours,” Chaiton says. “So the change in trends is that this is a social phenomenon. This is not that tobacco is causing this, it is something that has changed socially in the role of tobacco in society.”

Now that marijuana smoking has become more of a social norm, Chaiton says, programs aimed at keeping youth from risky behaviours such as drug abuse must take into account two factors: that more students now smoke marijuana compared to 30 years ago, and that students who smoke tobacco are more likely to use marijuana or other drugs and engage in at-risk behaviours such as vandalism and theft.

As marijuana use becomes more prevalent and socially acceptable, Chaiton says, the focus must turn to developing programs for youth that properly educate them on the risks.

Tobacco and marijuana are “similar drugs in many different ways,” Chaiton notes, and “people dramatically underestimate the risks associated with cannabis use, particularly among youth.”

“I would argue that we need to start talking about them in the same way and start addressing them in the same types of interventions,” he says, particularly given the growing public discussions about decriminalizing or legalizing marijuana.

“If we do legalize or change the regulations in dramatic ways, that does change the social environment again and that can, as we’ve seen a number of times, cause big shifts in youth and we could see another big shift in marijuana use among youth.”