Colorado opened its first pot stores in January, and adults in Washington state will be able to walk into a store and buy marijuana this summer. But this legalization of recreational marijuana is taking place without much information on the possible health effects.
"We should have been doing a lot more research to find out just how useful it is, how it affects the brain, et cetera, et cetera," according to Dr. Herbert Kleber, a Columbia University psychiatrist and drug abuse researcher.
But he says it's been hard to study the effects of Cannabis sativa, the plant that produces marijuana.
A large part of the reason marijuana research has lagged is that it's been very hard to get federal approval for such projects, as well as access to federally sanctioned supplies of the drug.
Kleber hopes that the push to legalize the drug will stimulate long-delayed research. Meanwhile, he thinks it's "a bad idea. I don't think we know what we're getting into."
He's not the only person who would like to see more science on marijuana and health. But not all think the drug poses big health risks, or at least not big enough risks to keep it illegal.
"Most cannabis users are moderate users," says Mark Kleiman, a professor of public policy at the University of California, Los Angeles, who studies drug policy. "They use a joint a week or less."
Jacquelene Cohen, the 29-year-old director of publicity for a Seattle publisher, is a typical user indulging once or twice a month, she says. "It's just like a small pleasure," she says, like "pouring a nice bourbon for a friend or passing a pipe with marijuana in it. It's just something nice to do when you're sitting around conversationally."
While Kleiman doesn't see enough health risks from marijuana to override the push toward legalization, he does worry about users who get hooked on the stuff. He says if the price of marijuana falls too much, it could result in more dependence, especially among adolescents and low-income users.
"The main risk of cannabis," he says, "is becoming habituated to cannabis and spending your whole life stoned."
About 11 percent of marijuana users fit the definition of dependence that is, their habit interferes with their life and they've been unable to cut back. That's lower than the dependence number for heroin and other opioids (23 percent), cocaine (17 percent), cigarettes or alcohol (15 percent) or nicotine (32 percent).
Heavy users aren't hard to find. "I have friends that I've never seen not stoned," Cohen says. "It doesn't seem that it's that harmful. My friends that smoke a lot of marijuana are productive and creative and sociable, and they form strong relationships and they make good life choices."
The National Institute on Drug Abuse says that while marijuana dependence is similar to that for other drugs, "the long-term clinical outcomes may be less severe."
Surveys show that 16 million Americans use marijuana at least once a month, and 2 million of those meet the criteria for dependence. Kleiman says that's not alarmingly high.
"It's not a very high risk," he says, "but it's a high risk if it's you or your child or your parent or your sibling. So people who say, 'Oh, cannabis isn't abusable, cannabis isn't addictive,' it seems to me just aren't looking at the data."
The addiction potential is higher among those who start using marijuana at younger ages. And the higher potency of marijuana these days may increase the risk of dependency, although data are lacking on that point.
Psychological treatment to wean marijuana users from dependency works about 70 percent of the time, according to Kleber, the Columbia University researcher. There's promising research indicating treatment can be improved with medication in particular a combination of synthetic THC (the main psychoactive ingredient in marijuana) plus a long-acting drug for high blood pressure only available in Europe.
When it comes to other adverse health effects, the evidence is unclear.
Marijuana smoke contains many of the same toxic chemicals as cigarette smoke. But even heavy marijuana smokers don't seem to have more lung cancer or emphysema.
"Oddly enough," Kleiman says, "the epidemiology has not come in to support the microbiology. The obvious speculation is that there's something else in cannabis that's actually a tumor-suppressing agent, and people are looking hard at that now."
Many worry that marijuana might increase the risk of schizophrenia. Authorities say there's enough evidence to warn against marijuana use for anybody who's had a psychotic episode or a family history of schizophrenia. Still, as millions of people have used marijuana in recent years, the incidence of schizophrenia has remained static, at around 1 percent of the population.
And then there's driving under the influence.
Kleiman says one study allowed heavy marijuana users to smoke as much as they wanted before testing them on a driving simulator.
Even when the subjects were "as stoned as they want to be," he says, they were about as impaired as people who just meet the drunken-driving threshold.
That's not entirely reassuring. But it suggests that marijuana might not be as dangerous as alcohol when it comes to driving.
But here's an important point: Kleiman says there's some evidence that marijuana users may be driving-impaired for hours after they think their high has worn off.
"That's where a real public health campaign could make a difference," Kleiman says, "saying if you smoke cannabis, do not drive for something like six hours after we'd have to do the research. That's a lot longer than the subjective high lasts. But it's probably what you need to get people actually safe."
Kleiman adds one more key point: "Cannabis is much more dangerous when it accompanies even a little bit of alcohol." So he thinks the blood-alcohol limit for drivers with any level of cannabis in their blood should be zero.
Of the states considering legalization, most are considering legalizing recreational marijuana (17 states) or decriminalizing possession of small amounts (12 states and the District of Columbia). Twenty states plus Washington, D.C., allow marijuana sale for medical use, although in some places the medical pretext is slim.