The Science of Pot

The lore is giving way to fact — and the news is mixed

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Jeff Riedel for TIME

It's fairly settled science that pot has analgesic and anti-nausea properties, which is why it can be so soothing for people undergoing chemotherapy or suffering from pain. But does that mean it's harmless, or are there more-compelling reasons to steer clear of the stuff?

Smoking joints does not seem to pose as many pulmonary risks as smoking cigarettes, in part because people simply smoke fewer of them. One large study in 2006 found no increased risk of lung cancer. But a French study last year found an "independent role of cannabis in the development of lung cancers." A 2009 study in the U.K. concluded that acetaldehyde, which is present in both marijuana and tobacco smoke, can cause DNA damage "with the possibility to initiate cancer development."

The science, however, remains unsettled, and other research has reached opposite conclusions. Another U.K. study showed that the anti-inflammatory properties of cannabis may have an anticancer effect. And a 2009 study in Spain found that tetrahydrocannabinol (THC), the main intoxicating component of pot, can lead human brain-cancer cells to self-destruct—though the study involved introducing the cells into mice and treating them with THC. That's a long way from treating cancer in a human brain.

The timeless question of whether pot makes you stupid seems easy to answer just by watching a stoner movie. Do those guys look smart to you? The science, though, is subtler than that, and the answer depends at least in part on the chemical makeup of what's in your stash. A recent British study looked at how users are affected by different ratios of THC and cannabidinol (CBD), pot's other principal intoxicant.

It's THC that's behind a pot high's otherworldly edge—and also behind the paranoia and hallucinations it can cause. It's CBD that gives the drug its sedating power. The lower the concentration of CBD relative to THC, the worse the subjects in the British study performed on word-recall tests; the higher the concentration, the better they did. Graded on the generous curve required when all the subjects were wasted, the people with the mellower buzz were generally smarter. While there is some conflicting data, most studies do not find lasting effects on cognition.

Although pot's rep for diminishing short-term memory is deserved, it surprisingly may have therapeutic value in treating Alzheimer's disease. A 2008 study in rats showed that the anti-inflammatory effect of a THC-like compound might slow the progression of the degenerative illness. The compound may also promote the growth of new cells in aging brains.

And what about addiction? Can you get hooked on pot? The answer is yes—depending on how addiction is defined. There are substance addictions and process addictions—heroin vs. sex or gambling, say—and they all target similar reward pathways in the brain. Heroin, alcohol and other substances, however, trigger violent withdrawal symptoms if the chemical is cut off, and that clearly doesn't happen with pot—to say nothing of sex and gambling. Even among experts, the existence of withdrawal symptoms used to be the key criterion defining addiction.

But that thinking has changed, and nearly everyone now goes by the broader definition of addiction in the Diagnostic and Statistical Manual—psychology's field guide to mental illness. The DSM describes addiction as compulsive use of a substance or repetition of a behavior despite repeated negative consequences. About 10% of pot users are addicts by that definition—lower than the roughly 15% to 30% of alcohol, tobacco, cocaine and heroin users who are addicts, but in the vicinity.

There may never be a final word on the health effects of pot, but if there were, it would not lie at the extremes. Somewhere between Reefer Madness and Alice's Restaurant is the truth.