Some 39.5 percent of high-school seniors view marijuana as harmful -- down from 44.1 percent just a year ago -- the National Institute on Drug Abuse (NIDA) said in its Monitoring the Future survey for 2013.
"That's 60 percent who think marijuana is not harmful!" exclaimed NIDA, a federal agency, on its blog for teenagers.
Moreover, nearly 23 percent of school seniors, typically 18-year-olds, said they had smoked marijuana in the month prior to taking part in the survey -- and 36 percent said they had done so in the preceding year.
But following referendums, the western states of Colorado and Washington are legalising the sale of the substance for recreational use, while 19 states allow the sale for medicinal use.
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With public opinion shifting, the Obama administration in August told federal prosecutors to cease targeting individual marijuana users in states where legalisation is in place.
Instead, it instructed them to focus their efforts on criminal gangs and sales to minors.
"Today's news demands that all of us recommit to bolstering the vital role prevention and involved parenting play in keeping young people safe, strong, and ready to succeed," he said in a statement.
Nora Volkow, director of NIDA, expressed concern not only at the number of teenage users in America, but also the kind of pot they are inhaling.
"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.