A new federal report shows that the percentage of American high-school students who smoke marijuana is slowly rising, while the use of alcohol and almost every other drug is falling.

The report raises concerns that the relaxation of restrictions on marijuana, which can now be sold legally in 20 states and the District of Columbia, has been influencing use of the drug among teenagers. Voters this year passed laws legalizing recreational-marijuana use in Colorado and Washington.

Health officials point to what they say is a growing body of evidence that adolescent brains are susceptible to subtle changes caused by marijuana.

“The acceptance of medical marijuana in multiple states leads to the sense that if it’s used for medicinal purposes, then it can’t be harmful,” said Dr. Nora Volkow, director of the National Institute on Drug Abuse, which issued the report. “This survey has shown very consistently that the greater the number of kids that perceive marijuana as risky, the less that smoke it.”

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Volkow said that one way marijuana might affect cognitive function in adolescents was by disrupting the normal development of white matter through which cells in the brain communicate.

According to the latest federal figures, which were part of an annual survey, Monitoring the Future, more than 12 percent of eighth-graders and 36 percent of seniors at public and private schools around the country said they had smoked marijuana in the past year. About 60 percent of high-school seniors said they did not view regular marijuana use as harmful, up from about 55 percent last year.

The report found that drinking was steadily declining, with roughly 40 percent of high-school seniors reporting having used alcohol in the past month, down from a peak of 53 percent in 1997. Abuse of the prescription painkiller Vicodin is half what it was a decade ago among seniors; cocaine and heroin use are at historic lows in almost every grade.

For the first time since the survey began, the percentage of students who smoked a cigarette in the past month dropped below 10 percent. Roughly 8.5 percent of seniors smoke cigarettes daily, compared with 6.5 percent who smoke marijuana daily, a slight increase from 2010.

Studies show that the concentration of THC in marijuana has tripled since the early 1990s, and Volkow said there was concern that increased potency could affect the likelihood of car accidents and could lower school performance.