Fewer teens are trying fake marijuana known by such names as K2 and Spice, apparently getting the message that these cheap new drugs are highly dangerous, according to the government's annual survey on drug use.

Synthetic marijuana is thought to have appeared in the U.S. in 2009, and soon after came a spike in emergency room visits, even deaths, as the drug caught on among young people.

About 8 percent of high school seniors said they've used some type of synthetic marijuana this year, according to the report released Wednesday by the National Institutes of Health. That's a sharp drop from the 11 percent of seniors who'd experimented with fake pot in 2012.

Plants used for medical marijuana. More teens are from fake marijuana, but usage of the real stuff remained steady, a the Monitoring the Future survey from the University of Michigan found.

Use of synthetic drugs among younger teens dropped as well — and fewer than 1 percent of students also are trying another new kind of illegal drug known as bath salts, said University of Michigan professor Lloyd Johnston, who heads the annual Monitoring the Future survey of more than 40,000 students in the 8th, 10th and 12th grades.

"The message has gotten out that these are dangerous drugs," Johnston said. "Their ever-changing ingredients can be unusually powerful. Users really don't know what they are getting."

Synthetic marijuana is made of dried plant material sprayed with various chemicals and packaged to look like pot. The Drug Enforcement Administration banned a number of chemicals used to make synthetic marijuana in 2011, but new chemical varieties continue to appear. Earlier this year, federal health officials discovered that two new types of fake pot had sickened more than 200 people in a month in Colorado.

The annual survey also found that teenage perceptions of the dangers of marijuana use continued to decline. In 1993, more than 60 percent of high school seniors considered marijuana dangerous, while this year less than 40 percent thought that.

The rate of use stayed steady, with 6.5 percent of high school seniors saying they regularly used marijuana in the past year.

The survey results were being released just weeks before recreational marijuana sales become legal in Colorado and Washington state for people over 21. Opponents of legalized marijuana long have said they worried about its impact on children.

Dr. Nora Volkow, director of the National Institute on Drug Abuse, said researchers worry that as perceptions of marijuana as a dangerous drug continue to decline use will keep increasing among teenagers.

Mason Tvert, communications director for the Marijuana Policy Project which advocates for regulating marijuana, said steady rates of marijuana use among teenagers "underscores the benefits of regulation versus prohibition."

Marijuana remains illegal under federal law, but the Justice Department in August pledged not to target the marijuana industry in states where the drug has been legalized as long as the states keep pot away from children, other states, criminal cartels and federal property. While only two states have legalized the production, sale and use of recreational marijuana, 18 others and the District of Columbia allow medical marijuana.