Although teen smoking rates are at a record low, more of them are smoking pot and fewer than ever believe it is bad for them. Data released last December as part of the National Institute on Drug Abuse's Monitoring the Future project show that only 44.1 percent of 12th graders believe regular marijuana use is harmful, the lowest level since 1973. That may explain why more than one third of high school seniors tried pot in 2012, and one in 15 smoked it daily.

The growing acceptance of medical marijuana may be behind teens' changing attitudes. Since 1996, 18 states, plus the District of Columbia, have made it legal for adults to obtain pot with a doctor's prescription. And last November, Colorado and Washington became the first states to legalize marijuana for anyone older than 21 years. “This shift in perceived risk may very well have resulted from the widespread endorsement of medical marijuana use,” says Lloyd Johnston of the University of Michigan, who led the Monitoring the Future project.

But pot poses a higher risk for teens than for adults. In August investigators at Duke University and other institutions published the results of a 25-year study suggesting that heavy use among adolescents can do permanent cognitive damage. Subjects who were diagnosed with marijuana dependence as teens and adults suffered IQ declines of up to eight points between the ages of 13 and 38, even after the researchers controlled for other drug dependence, schizophrenia and education. (Abstainers' IQs rose slightly.) Moreover, the IQs of teen users did not recover even if they quit in adulthood.

How much marijuana is too much? “It's hard to find out,” says lead study author and Duke clinical psychologist Madeline Meier. There is no accurate way to measure consumption because marijuana joints are rarely identical and potency varies. What is clear is that adolescent brains are particularly vulnerable to marijuana's effects, so teens would be smart to abstain—and may stay smarter for it, too.