Reefer madness? Study appears to dispel theory of spike in teen marijuana use in states where it's legal

Fears of a jump in U.S. high school students using marijuana in states where recreational cannabis has been legalized appears to be contradicted by a new study.

Weed consumption among high school teenagers has actually declined in states where pot is legal, the study published this week in the medical journal JAMA Pediatrics concludes.

"Consistent with the results of previous researchers, there was no evidence that the legalization of medical marijuana encourages marijuana use among youth," researchers from Montana State University and San Diego State University wrote.

Eleven states and Washington, D.C., have legalized recreational use of marijuana, but consumers generally must be at least 21-years-old to buy cannabis at dispensaries or possess it. A total of 33 states have legalized medical marijuana.

The study found an 8% drop in the number of high school students who said they used marijuana in the last 30 days. The researchers also found a 9% dip in the number of students who said they'd used marijuana at least 10 times in the last 30 days.

The authors of the study reached their conclusions by culling through the biennial Youth Risk Behavior Survey given to 1.4 million U.S. high school students between 1993 and 2017 by the Centers for Disease Control and Prevention (CDC) that included questions on marijuana use.

The authors noted in their report that "policymakers are particularly concerned that legalization for either medicinal or recreational purposes will encourage marijuana use among youth," but found nothing in their research or in an analysis of previous research to support that concern.

The researchers also suggested that the decline in marijuana use among high school students may be partly due to the fact that drug dealers are being replaced by licensed dispensaries that require proof of age.

The authors reported that the results of their study mirror previous studies showing a decline in teen use after sales of recreational pot began in 2014 in Washington state.

But Linda Richter, director of policy research at the nonprofit Center for Addiction in New York, pointed out that while the researchers used legitimate CDC surveys of high school students to draw their conclusions, the surveys were based on those given to students in just seven of the 11 states that have legalized marijuana. She said the study didn't include surveys given to students in Oregon and Washington State, two states with the longest histories of legal weed.

"It defies logic to argue that more liberal recreational marijuana laws somehow help to dissuade young people from using the drug," Richter told ABC News on Wednesday.

Richter also took issue with the suggestion that legalization has put illegal marijuana dealers out of business, arguing that the "black market for marijuana is alive and well" and that weed is generally cheaper on the streets than in the dispensaries.