The study, conducted by researchers at Johns Hopkins University Bloomberg School of Public Health, found that in the past 15 years, marijuana use among high school students has decreased significantly — even though this same time period has seen a lot of change in drug policy. The researchers used data from the National Youth Risk Behavior Survey, which examined answers from over 115,000 teenagers in grades 9-12, between 1999 and 2013.

In 1999, 51 percent of boys and 43 percent of girls had used marijuana, and those numbers dropped to 42 percent and 39 percent, respectively, by 2013. In addition to a drop in weed use, the researchers observed a decrease in other illegal drugs among teens, such as hallucinogens and cocaine. The use of cigarettes and alcohol has also fallen among youth, facts backed up with previous research. The researchers also found that the gap between male and female use has been decreasing.

With decriminalization in many states, medical marijuana available in other states, and a few with legal recreational weed as well, the drug is far more available than it used to be. Then why are kids using it less?
The researchers aren’t entirely sure. What’s confounding, however, is that other recent studies have pointed to an increase in marijuana use while alcohol and cigarette use is dropping. One Penn State study that analyzed weed use among high school students from 1976-2013 concluded that while trends peaked in the 70s, then dropped in the 90s, they were beginning to rise again.

The Johns Hopkins researchers note that rates increased in the late 90s, and have been dropping since, but began seeing a slight increase in 2009. More research will need to be conducted to estimate whether this small rise is a blip or a greater sign that availability is boosting marijuana use.
“People have been very quick to say that marijuana is going up and up in this country, particularly now that marijuana has become more normalized,” Renee Johnson, assistant professor in the Department of Mental Health at the Bloomberg School, said in the press release.

“What we are seeing is that since 1999 — three years after medical marijuana was first approved — the rates of marijuana use have actually fallen. But we will be watching those states where recreational marijuana use has been legalized to see if that leads to increased use among teens.”
States that have legalized some medical and recreational use of marijuana include Colorado, Oregon, Alaska, Washington, and the District of Columbia.

It’s too soon to tell whether widespread availability will impact teens, but Johnson notes it’s still important to keep in mind how smoking weed in early years can negatively impact brain development and memory — and thus should be avoided until the legal age, 21.