Researchers gathered data on the frequency of cannabis use among 3,725 students from Australia and New Zealand, and then looked at the students' developmental outcomes up to the age of 30. They found "clear and consistent associations between frequency of cannabis use during adolescence and most young adult outcomes investigated, even after controlling for 53 potential confounding factors including age, sex, ethnicity, socioeconomic status, use of other drugs, and mental illness."
Significantly, they found that the risks for negative outcomes increased with the frequency of cannabis use. In a conference call, study co-author Edmund Sillins said that the relationship between cannabis use and negative outcomes is significant even at low levels of use (e.g., less than monthly), and that "the results suggest that there may not be a threshold where use can be deemed safe" for teens.
I charted the findings below. According to the study, there are significant relationships between cannabis use and high school graduation, college graduation, suicide attempts, cannabis dependency (not wholly surprising), and other illicit drug use.
The chart plots the odds ratios of the frequency of cannabis use on various outcomes, compared to not using cannabis at all. A value of 1 would indicate equal odds of a given outcome, 2 would indicate an outcome twice as likely as you'd get from not using cannabis at all, and a value less than 1 would indicate decreased odds of a given outcome.
So for instance, a person who uses cannabis less than monthly would have slightly lower odds of graduating high school or getting a college degree, compared to a person who doesn't use at all. Increased use further decreases this likelihood. On the other hand, a person who uses cannabis monthly would have roughly 4 times the likelihood of becoming dependent on cannabis as a person who doesn't use at all.
It's worth noting that for many of these indicators, the confidence intervals (error bars on the chart) are fairly broad, meaning that there's some degree of imprecision in these numbers.
You can expect these findings to be highly cited by opponents of liberalized marijuana laws, like the Office of National Drug Control Policy and the Smart Approaches to Marijuana project. But it's important to put them in proper context.
First, the causality isn't 100 percent clear. The researchers did a fantastic job of trying to account for a number of confounding factors. But particularly when it comes to the educational outcomes, there are a lot of factors at play. For instance, if a teacher knows or even suspects that a certain kid is using drugs, that may predispose the teacher against that student. "Teachers are very likely to stigmatize drug users," says Joseph Palamar, co-author of another recent study comparing teen marijuana and alcohol use. "That stereotype gives kids problems, and that kid's not gonna want to go to class."
Palamar also says that because marijuana "is an illegal drug, you have to buy it in an illegal manner, and then you’re exposed to the black market. Marijuana use is affiliating you with other kids, some of whom might be problematic – people more likely to question authority. You become affiliated with things that might have a negative impact on your education."
Moreover, Palamar's research shows that because of marijuana's legal status, teen cannabis users are much more likely to get into trouble with the police than teen alcohol users. And in many cases, if you have a drug conviction on your record, you become ineligible for college aid. "If you get caught with drugs, you're not able to go to college," he told me.
In other words, many of the problems associated with teen cannabis use are likely a function of the drug's illegal status.
That said, it's completely reasonable to accept that heavy use of any drug as a teenager - be it weed, alcohol, or tobacco - is going to lead to negative consequences down the line. This in itself isn't an argument for prohibition of cannabis. For that, you'd need to demonstrate a link between relaxed cannabis laws and increased teen drug use, with the negative social consequences that go along with it. But multiple studies have shown a flat or even negative relationship between medical marijuana laws and teen cannabis use, for instance. And very early data out of Colorado shows a slight dip in teen use since the state passed a legalization measure in 2012.
But the study does lend strong support to the case for efforts to keep the drug out of the hands of teens, and it does make a case for closely monitoring adolescent marijuana use in states that do legalize it.
Thinking more broadly about the effects of drug prohibition, you have to weigh the consequences of teen drug use against the societal consequences of failed drug war policies - skyrocketing incarceration rates and countless families and communities decimated by the illegal drug trade and its overzealous opponents.