High schoolers now report using marijuana more than cigarettes or cigars, according to a US Centres for Disease Control and Prevention report released Oct. 15.

In 1997, 21% of high schoolers said they had smoked a cigarette or cigar on one or more days in the last 30 days. By 2013 (the newest data available), only 7% said they had, representing a remarkable 64% decrease.

For marijuana in 1997, only 4% of students said they had used it once or more in the last 30 days. By 2013, that had climbed to 10%.

The authors of the CDC report said this increase in use was probably at least in part because only 40% of 12th graders now think smoking marijuana regularly is harmful — roughly the same percentage as in the 1970s. Fear of marijuana seemed to peak in 1991, when 79% of 12th graders said regular use was harmful, but it’s been declining ever since.

Extended, heavy use of marijuana has known detrimental health effects, including respiratory problems, impaired cognitive function, and the exacerbation of certain mental illnesses. But scientific evidence is starting to support, in part, teens’ changing perceptions.

A study published earlier this year in the journal JAMA Psychiatry found marijuana didn’t cause certain regions of the brain to shrink, contrary to popular belief. (The brain effects of extremely heavy use, the authors noted, “deserve further study.”) Research has also shown that the perception of marijuana as a “gateway drug” is largely misguided, and that it may be safer than either alcohol or tobacco cigarettes.

The CDC report, based on the Youth Risk Behaviour Surveys, asked anywhere from 11,000 to 13,000 students in grades nine through 12 about their experiences using cigarettes, cigars, or marijuana.

Because the surveys are self-reported, the numbers are likely lower than the actual percentage of high schoolers who smoke. They didn’t ask about hookahs or e-cigarettes, either, which could also affect the usage numbers.

Marijuana is still listed federally as a Schedule I substance, and the CDC report refers to it as a “gateway drug.”

But during the time the surveys cover, 23 states and Washington, D.C. have legalised medical marijuana. In 2012, Washington state and Colorado passed laws making small amounts of the substance legal, though those statutes conflict with federal law. Many other states, such as California, have significantly reduced the penalties for possessing small amounts of marijuana.

The CDC report said these changing laws are another reason why marijuana use among teens may have risen, since they add to the perception that the drug isn’t harmful.

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