As pot activists fight for full legalization, the drug warriors who oppose them have long sought their own elusive grail: conclusive, scientific proof that marijuana poses significant health risks. The only problem? An unshakable dependence on flawed studies. "Much of what we believe in this society about drugs is just bullshit," says Columbia University professor Carl Hart. "And we as scientists have been complicit." Luckily, there's a new generation of researchers pushing back against these oft-repeated prohibitionist canards.
MYTH: Pot ruins the teenage brain
This drug-warrior talking point has gained such traction recently that even respectable liberals like New York Times columnist Bill Keller regurgitate it as a self-evident truth. The only basis for this are shoddy reports like a recent international study purporting to link adolescent marijuana use with long-term IQ decline. "Do the authors believe that cannabis is the only thing that could affect your IQ between the ages of 13 and 38?" asks Norwegian economist Ole Røgeberg. "That puzzled me."
MYTH: Pot can make you schizophrenic or depressed
Other studies have tried to engineer even bigger scares, claiming to demonstrate a connection between smoking pot as a teen and serious mental illnesses like depression or schizophrenia later on. "Give me a break," says Hart. "You look at these studies and see how they determine psychosis – it's a joke." The same old logical fallacies show up here, too. "Which came first?" asks Suzi Gage, a Ph.D. student at the University of Bristol. "Did the cannabis use precede the mental-health problems, or is it that people who are psychotic use cannabis to self-medicate?"
MYTH: Pot destroys your memory
Hart's research actually suggests the opposite: Temporary short-term memory loss was worst among inexperienced smokers, but regular stoners showed no serious decrease in cognitive function. "Just like any other drug, if you let tolerance develop, you decrease a lot of the negative effects," he says.
MYTH: Pot is a gateway to harder drugs
Anti-drug zealots want you to think that if your honors-student daughter takes a couple of hits off a joint over spring break, she'll probably be passed out in an alley with a needle sticking out of her arm by Labor Day. "Most people who try it don't even continue smoking marijuana," says Hart. "You might as well argue that pot is a gateway drug to get in the White House."
MYTH: Pot is dangerous even in moderation
Columbia University professor Meg Haney has been studying marijuana addiction since the Nineties, focusing on the relatively small fraction of smokers who do become dependent. The good news for everyone else: "If somebody is smoking one or two joints a week," says Haney, "the consequences are probably going to be very, very, very minimal."
MYTH: Pot has no medical benefits
Dismissing the very idea of marijuana as medicine is key to the prohibitionist agenda. Leaving aside years of anecdotal and clinical support for pot's effectiveness in relieving chronic pain, nausea, glaucoma and more, there is fresh evidence that the active compounds in marijuana can actually kill cancer cells. Guillermo Velasco, a professor at Madrid's Complutense University, has found that THC has a powerful tumor-shrinking effect in rodents with breast, liver, pancreatic and brain cancers. Next, he hopes to test the hypothesis on people. "I think we'll see clinical trials in the next five years," says Velasco. "Cannabinoids also ameliorate the side effects of chemotherapy, so they could make patients feel better in general."
This story is from the June 20th, 2013 issue of Rolling Stone.