College students in the United States are using more marijuana than in previous years, according to a new study. Last year 38 percent of college students said they used marijuana in the past year, up from 30 percent in 2006.

Use of other drugs, including opioids and amphetamines, declined among college students, HealthDay reports. The findings come from the Monitoring the Future study, conducted by the University of Michigan Institute for Social Research.

More students may be using marijuana because of a decrease in perceived risk, the researchers say. “This increase in use and decrease in perceived risk of harm regarding marijuana use should be taken seriously by college administrators, parents and students themselves,” study co-lead researcher John Schulenberg said in a news release.