It looks as if more American teens are just saying no.

Fewer teens say they’re using drugs and alcohol, with drug use down across the board of various substances, an annual study conducted by the University of Michigan on behalf of the National Institute on Drug Abuse has found.

In the study, whose respondents anonymously replied via paper questionnaires during classroom hours, some 45,000 students nationwide were asked whether they had tried various substances in their lifetimes, in the last year, and in the last 30 days.

On all three questions, the number of eighth and 10th graders saying they had tried marijuana was down year over year.

Among high school seniors, the percentage saying they had tried marijuana in the last year rose slightly, from 34.9 percent to 35.6 percent. But the number of eighth graders saying they had used marijuana was the lowest it has been since 1993.

Synthetic marijuana is also less popular. Just 2.7 percent of eighth graders said they had tried it, down from 3.1 percent last year, and 3.5 percent of 12th graders said they had, down from 5.2 percent.

The decline in the share of teens who say they’ve used marijuana comes as a growing number of states have legalized the drug for medical and recreational use. Last month, voters approved ballot measures legalizing marijuana for recreational use by adults in California, Nevada, Massachusetts, and apparently Maine; voters in a handful of other states voted to legalize its medical use, too.

Advocates of marijuana legalization were pleased with the new study’s findings, since opponents have argued that legalization could cause underage drug use to rise. “The best way to prevent teen marijuana use is education and regulation, not arresting responsible adult consumers and depriving sick people of medical marijuana,” Mason Tvert, the communications director for the Marijuana Policy Project, said in a statement.

The study’s researchers attribute the decline in teen marijuana use to a drop in tobacco use.

“There’s long been a debate within the field about tobacco and cigarette smoking as a gateway effect,” said Richard Miech, the incoming principal investigator of the study, which has been conducted for the last 42 years. “Once kids don’t smoke ever in their life, and we’ve seen substantial declines, it seems like they don’t progress to the other drugs.”