The 2013 Monitoring the Future Survey also found the percentage reporting daily use of marijuana has edged up. The National Institutes of Health surveys students in the eighth, 10th and 12th grades for the report.
Only 39.5 percent of high school seniors said using marijuana regularly is likely to be harmful, the report said, down from 44.1 percent last year. In the past, students were far more likely to believe marijuana use was dangerous.
Among seniors, 6.5 percent reported daily marijuana use, up from 6 percent in 2003 and 2.9 percent in 1993. In the 10th grade group, 4 percent reported daily use.
More than one third, 36 percent, of seniors said they had smoked marijuana at least once in the past year, and 23 percent had done so in the past month.
Dr. Nora Volkow, director of the National Institute on Drug Abuse, said the problem goes beyond increased use.
"It is important to remember that over the past two decades, levels of THC -- the main psychoactive ingredient in marijuana -- have gone up a great deal, from 3.75 percent in 1995 to an average of 15 percent in today's marijuana cigarettes," she said. "Daily use today can have stronger effects on a developing teen brain than it did 10 or 20 years ago."
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