Momentum toward public acceptance of marijuana is building. Last year Colorado and Washington became the first two states to legalize pot for recreational use. Alaska and Oregon may soon follow suit, and many other states are considering legislation to decriminalize use of the drug or make it legal for medical purposes. Earlier this year, President Obama himself noted that while he considered the drug a "bad habit and a vice," he did not think it any more dangerous than alcohol.

But how well do we really understand the health impacts of marijuana? At a private dinner party for journalists last week hosted by National Geographic and USA Today, NIH director Francis Collins voiced concerns: Studies have shown that prolonged use of the drug by teenagers can lead to permanent decline in cognitive ability, and the possible contribution of pot smoke to lung cancer is unknown. "We don't know a lot about the things we wish we did," Collins said.

To follow up on Collins's remarks, National Geographic interviewed Nora Volkow, director of the National Institute on Drug Abuse (NIDA).

What are your concerns about marijuana as legal sales begin in Colorado and Washington?

We're keeping an eye on a possible increase in use, particularly among young people. And there's concern about the fact that you're creating an industry whose purpose is to sell marijuana products. The more people who smoke, the more profit they generate. When you have a profit incentive to promote the use of a substance, that could increase the problems associated with it.

President Obama recently compared marijuana to tobacco and alcohol, two unhealthy but legally tolerated vices. Does marijuana belong in that category?

I don't like to say one drug is better or worse than another; each must be viewed within its own context. Tobacco is clearly the number one killer among drugs. On the other hand, when you smoke a cigarette, it doesn't impair your brain's cognitive capacity. That's very different from drinking alcohol or smoking marijuana.

Alcohol can disrupt your coordination; that's why it's so frequently associated with car accidents. It can also make you more impulsive. Marijuana, by contrast, drops your mental state; it makes you slower and interferes with your capacity to learn and memorize. For a young person, whose main responsibility is to learn and study, that can be very disruptive and have different consequences than cigarette smoking. That's why I say each [drug] has to be considered within its own context.

A number of studies have indicated that marijuana smoking holds far greater risks for teens than for adults, especially in terms of brain development. There's also concern over a possible connection between pot smoking and the onset of schizophrenia. But there are classic correlation-or-causation questions with these studies. What's your reading of the situation?

That's a difficult one. Establishing causality with mental illness is not clear cut. What the research seems to show—for schizophrenia, depression, and anxiety—is that if you have an underlying predilection for these illnesses, smoking marijuana may accelerate the disease's progress and exacerbate it. Studies have found that schizophrenics are more likely to smoke marijuana, but that doesn't mean that marijuana produced the schizophrenia.

In any normal person, if you give them high enough doses of 9-THC—delta-9 tetrahydrocannabinol, marijuana's major psychoactive ingredient—you may trigger paranoia. But it's generally short lived. In young people who have a vulnerability to schizophrenia, by contrast, marijuana can trigger a chronic psychosis, which is a much more serious condition. So there may be a connection there. It's a difficult question, because other factors may also be at work. Many people with schizophrenia may also have started smoking cigarettes at an early age, yet nobody suggests cigarettes trigger schizophrenia. In certain people who have genetic vulnerabilities, marijuana use might be a contributing factor for triggering a mental illness. I'm not completely ruling that out.

NIH Director Francis Collins recently touched on the need for more studies on marijuana smoking and lung cancer. What little research we have suggests a much lower risk of lung cancer among marijuana smokers as compared to tobacco smokers. What are the risks to a pot smoker's lungs?

The literature seems to indicate that if you're a heavy user of marijuana, that may be associated with an elevated risk of cancer of the lung. But it has to be very heavy use. For milder use, the risk seems to be much lower. There is some evidence of a heightened risk of testicular cancer, though, which is a fairly rare form of cancer.

The issue with marijuana and the lungs is that when you're smoking it, you're inhaling a wide variety of chemicals other than THC. Now, a tobacco smoker may smoke 20 cigarettes a day, whereas a very heavy marijuana smoker will smoke at most five joints a day. So the amount of exposure to all those chemicals is much greater in tobacco smoking than in marijuana smoking.

Medical marijuana advocates and federal officials do not agree on whether smoking the drug is a safe way to deliver its potential benefits. If THC and CBD have potential medical value, what are the risks of smoking the raw plant?

Some researchers are interested in the therapeutic value of THC and cannabidiol (CBD), another promising compound found in marijuana. But smoked marijuana is problematic as a medicine. You don't want to deliver cancer-causing compounds along with potentially therapeutic compounds.

One of the things technology and science allow us to do is investigate and isolate the active ingredients in a plant like marijuana, and then deliver them to the patient in a way that minimizes side effects. A doctor may have a patient for whom the dumbing-down effect of smoked marijuana may be very adverse. You want to give them the benefits without the harm. This is exactly why we develop medicine.

Heroin is a perfect example. It's a very potent narcotic. But when we're treating pain, we don't give heroin. We develop medications that can be just as effective but don't have the side effect of addiction.

Recently there have been calls to rethink marijuana's classification as a Schedule I drug. Is it time to consider rescheduling it?

Well, that's for another agency to decide. At NIDA, we do the research and provide the evidence that other agencies use to make their policy decisions. My view is that the cannabinoids are one of the most fascinating targets we have for the development of medicines. It's an extremely important area of research. As research starts to emerge showing the possible health benefits of specific compounds within marijuana, like 9-THC or cannabidiol, one could ask if it's appropriate. I think that ultimately the data will determine whether it should be reconsidered or not.

State regulators in Washington and Colorado have been frustrated with the lack of research into marijuana's effect on the body and mind. They're setting limits on THC content in edible products, but have little science to guide them. Should we be doing more research on the basic effects of marijuana on the body and brain?

I think we want that information. We need to do those studies and know more about the consequences of marijuana use. So many people want to polarize the issue: Marijuana is perfect, it cures everything; or it's evil and destroys everything. It's more complex than that. That's why we need science to come in and address these questions.

Are you optimistic or pessimistic about what might happen in Colorado and Washington State? What indicators are you watching?

My prediction is that state legalization will expose more people to marijuana. As a result you're going to have many more adverse consequences, just as we have with nicotine and alcohol—simply because so many more people will become exposed to it.