THURSDAY, June 20, 2019 (HealthDay News) -- As society relaxes its rules around marijuana, U.S. teens seem to be responding by using the drug in much bigger numbers than a generation ago, new research shows.

The study looked at 1991-2017 U.S. federal health data on more than 200,000 high school students. It found that the number who said they'd used pot at least once over the past month rose 10-fold -- from 0.6% in 1991 to 6.3% by 2017.

Many are becoming "dual users" of both marijuana and alcohol: The number of teens who admit to using both substances at least once a month has almost doubled -- from 3.6% in 1991 to 7.6% in 2017.

Why these big changes? Study author Hongying Dai believes changing social mores and legislation has been a big factor.

"Public opinion on marijuana use have changed dramatically, and restrictions on marijuana use have been relaxing," wrote Dai, of the University of Nebraska's College of Public Health in Omaha. "Currently, 33 states and the District of Columbia have laws in place that allow marijuana to be used medically, recreationally, or both."

There was some good news from the study -- teens are increasingly turning away from cancer-causing cigarettes and other combustible tobacco. According to the study, teen smoking has dropped from 4.4% of high school students in 1991 to just 1.3% today. Teens are drinking much less, too, with alcohol use falling from about 24% of teens in 1991 to 12.5% in 2017.

Still, the "surge" in marijuana use is troubling, Dai said, and "highlights the importance of marijuana prevention among youths."

She added that usage rates increased most dramatically among black and Hispanic youth. Over the time period covered by the study, use of marijuana by black teens soared from 2% to 13.5%, and from less than 1% to nearly 9% among Hispanics. In comparison, in 1991 0.3% of white teens said they'd used pot over the past month, and 3.7% said so by 2017.

Dr. Scott Krakower helps direct psychiatry, including addiction issues, at Zucker Hillside Hospital in Glen Oaks, N.Y. He wasn't surprised by the marijuana findings, saying that "with diminishing perceptions that it is harmful," of course, youth are turning more readily to the drug.