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Study: Teens who smoke pot daily are 62 percent less likely to finish college

Another study has found strong correlations between adolescent marijuana use and a range of bad outcomes, including a lower chance of completing high school and a greater likelihood of trying other illicit drugs.

The analysis — one of the biggest of its kind on the issue of marijuana — looked at data from three large, long-running longitudinal studies in Australia and New Zealand. Researchers found that, after controlling for 53 factors ranging from socioeconomic differences to mental health, young adults who used marijuana more frequently before age 17 were more likely to have worse outcomes by age 30 for high school completion, university degree attainment, suicide attempts, and other illicit drug use. They found no correlation between adolescent pot use and depression or welfare dependence.

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As one of the biggest studies of its kind, the research provides more ammunition to those who view adolescent marijuana use as a big risk. The concern is the primary reason states undertaking marijuana legalization, particularly Colorado, are launching big campaigns to curtail teenage drug use. Almost everyone, including legalization advocates, agrees teens should probably stay away from pot.

But a major caveat in the study is that correlation doesn't equal causation. It's possible that young adults who don't complete high school tend to gravitate to marijuana, since it's a relatively cheap, accessible form of entertainment. Or perhaps people who use harder drugs are simply more likely to use pot on a daily basis to supplement other drug use, not that marijuana use leads to harder drug use.

Previous research from the RAND Corporation, for instance, suggested marijuana is popular relative to other drugs perhaps because pot is cheaper and more accessible. If harder drugs were more accessible, then marijuana could lose its popularity among high school dropouts and harder drug users in favor of other substances.

In that sense, the study doesn't add much to what public health experts know about marijuana. Prior studies have correlated adolescent pot use to faults in cognitive development. The question for researchers has always been whether that correlation equals causation — and this latest study, despite its strong controls, doesn't provide a complete answer.

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