Marijuana use among U.S. college students rose 22 percent over the past decade, according to a study released this week by the Harvard School of Public Health. And "very competitive" colleges in the Northeast posted the largest increases.

Last year, 15.7 percent of students surveyed claimed to have used marijuana in the past month. In 1993, the rate was 12.9 percent.

"These new findings should be a source of concern for those involved with the prevention and treatment of illicit drug use among young people," Lecturer on Social Psychology Henry Wechsler, one of the authors of the report, said in a press release.

Wechsler is also director of the College Alcohol Study, a School of Public Health study whose first report was released six years ago and of which the marijuana study is a part.

The authors of the report say the study's results may mark the end of a 20-year decline in drug use.

"This trend threatens to slow or reverse the decline in illicit drug use among adults in the United States since 1980," the report reads.

But prevention cannot begin at the university level.

"[We] need education programs for younger students as well as programs in college," Weschler wrote in an e-mail message. "The younger one begins to use drugs, alcohol or tobacco, the greater the problems."

Colleges classified as "very competitive" showed higher usage rates and larger increases over the past seven years than college classified as "competitive," the report found.

Some Harvard students say that's not surprising.

"Students who go to more competitive colleges are more likely to be open-minded, so they're more likely to experiment than to just say no," said Keziah R. Cook '04.

But experts say it's the high-pressure environment that's more likely to be the cause of higher marijuana usage rates at competitive schools.

"Stress is an influence in people's decisions around substance use," University Health Services Educator Michael A. Hoyt said.

One student said Harvard is no exception to national trends.

"I'll smoke [marijuana] every once in a while--it's just a chill thing that a lot of people do," said the Eliot House sophomore, who asked that his name not be used. "Harvard is conducive to it, since people here are pretty affluent and have a lot of access and just do it."

But not all students think marijuana use on campus is so prevalent.

"I'm kind of surprised by [the results of the study], since it seems like not that many people at Harvard use marijuana," Cook said. "It's obviously here but it doesn't have nearly as much of a presence as alcohol."

The study found that marijuana use increased among most demographic groups, although it was highest at colleges in the Northeast and lowest at Southern colleges.

Usage was higher among white non-Hispanics than any other ethnicity, and men used marijuana more than women, according to the study.

The report was based on the responses of more than 14,000 students at 119 four-year colleges in 40 states.