The problem is that marijuana is not, in fact, “harmless.” (Update: After publication, the Marijuana Policy Project altered the language on its site to call pot “a much less harmful substance” than alcohol. Here is a screenshot of the original.) Proponents are spinning the science — casting pot as a threat only if used improperly, much like a car — for the sake of advancing their political agenda. It’s fine for people to believe the government has no business conducting a “war on drugs,” but it’s something else entirely to trivialize or simply deny marijuana’s harmful effects. These dangers are real, according to a recently released comprehensive review of 20 years of scientific literature from Wayne Hall, who advises the World Health Organization on addiction and runs the University of Queensland’s Center for Youth Substance Abuse Research. And the dangers need to be dealt with.

The role of government in regulating drugs can’t be separated from what those drugs do to people. On that question, pot-libertarians have taken too many liberties — about both its medical and its recreational effects.

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Proponents claim marijuana can slow or stop damage caused by glaucoma (by lowering eye pressure), but as Henry D. Jampel, professor of ophthalmology at Johns Hopkins, points out, marijuana could actually worsen the vision loss caused by glaucoma. “Although marijuana does lower the eye pressure, it also lowers blood pressure,” he notes. “Lower blood pressure could result in reduced blood supply to the optic nerve, which in turn might harm the optic nerve.”

Beyond the medical applications, legalization proponents argue that marijuana should be available for recreational use because it’s “harmless” and not addictive. But the active ingredient in marijuana, THC, is a powerful psychoactive ingredient that can cause hallucinations or delusions and interfere with the way the brain makes and stores memories. Hall’s study found that marijuana use doubles the risk of developing psychotic disorders, including schizophrenia. He also found that one in 10 adults who regularly smoke the drug become dependent on it, and those who use it are more likely to go on to use harder drugs.

His study also emphasizes the danger that marijuana use poses to teens — one in six teenagers who regularly smoke pot will become dependent. Other research backs up these warnings: A recent review of more than 120 studies found that teen marijuana use is associated with subsequent addiction to other drugs and psychosis, such as schizophrenia. And a recent study from researchers at the University of Wisconsin at Milwaukee linked regular marijuana use by teens to drops in IQ points and to memory problems.

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While the states that have legalized marijuana completely have done so only for residents 21 and older, peer-reviewed research has linked heavy marijuana use with long-term memory problems and other health effects in adults. Marijuana also poses dangers to pregnant women, with studies linking the drug to lower birth weight, impaired brain development and behavior problems in adolescents.

And while proponents of legalization point out that it’s impossible to overdose on cannabis, that doesn’t mean there aren’t any instances of marijuana-related deaths. A handful of deaths in Denver were tied to edible marijuana use this year, not to mention the increased risk of fatal car accidents due to drivers impaired by marijuana. Hall’s study analyzed epidemiological studies and laboratory evidence from a number of researchers and concluded that driving after smoking pot approximately doubles the risk of a car crash.

Because marijuana has been illegal for decades, intensive studies are lacking and there are still many questions surrounding the safety of marijuana. Hall’s report is a meta-analysis of available research on marijuana, but much of the work on marijuana’s effects has focused on heavy, long-term users or vulnerable populations, including teenagers and pregnant women. We need more research on the effect of marijuana on casual users.

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