More U.S. teens are using marijuana and see it as less of a risk, while their alcohol use has dipped to historic lows, according to an annual national survey of drug use released Wednesday.

The findings are based on an annual survey of 47,000 teens conducted by the University of Michigan for the National Institute on Drug Abuse.

One of every 15 high school seniors reported smoking marijuana on a daily or near daily basis, the highest rate since 1981.

"One thing we've learned over the years is that when young people come to see a drug as dangerous, they're less likely to use it," Lloyd Johnston, the study's principal investigator, said in a telephone interview with The Associated Press. "That helps to explain why marijuana right now is rising, because the proportion of kids who see it as dangerous has been declining."

The study said marijuana use among teens rose in 2011 for the fourth straight year after considerable decline in the preceding decade.

One of every nine students in their last year of school before college reported using synthetic marijuana within the previous 12 months.

The synthetic drug survey question was asked for the first time this year. Fake marijuana, sometimes sold in drug paraphernalia shops and on the Internet as incense, contains organic leaves coated with chemicals that provide a marijuana-like high when smoked.

A Drug Enforcement Administration emergency order banning the sale of five chemicals used in herbal blends to make synthetic marijuana took effect March 1. Many states also have their own laws banning the sale of synthetic marijuana.

White House drug czar Gil Kerlikowske called on parents to get involved to help stop the use of synthetic marijuana.

"It's not in the vocabulary of parents, and they need to be aware of it so that when they have that conversation about substance abuse that they are knowledgeable and they talk about this," he told the AP.

Alcohol use continued a trend of decline dating to the 1980s and hit a historic low for the survey, which began in the 1970s.

Other drugs showing some evidence of decline in use this year include cocaine, crack cocaine and inhalants.

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The survey can be found at http://www.monitoringthefuture.org