Despite its greater availability, marijuana use among American high school students is significantly lower today than it was 15 years ago, according to a recent study from Johns Hopkins Bloomberg School of Public Health.

Although marijuana use is significantly greater than the use of other illegal drugs, with 40 percent of teens in 2013 saying had smoked marijuana at least once, that figure is down from 47 percent in 1999, Examiner reported.

Marijuana policy has undergone significant changes over the past 20 years. Since 1996, 34 states have passed legislation removing criminal sanctions for medical use of marijuana. Eleven states have passed laws decriminalizing possession of small amounts of marijuana, adding to nine that passed such laws in the late 1970s. Four states have passed laws allowing for the recreational use of marijuana for people over 21.

"People have been very quick to say that marijuana use is going up and up and up in this country, particularly now that marijuana has become more normalized," Renee M. Johnson, who led the study, said in a statement. "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."

For the study, researchers examined data from the National Youth Risk Behavior Survey, a biennial school-based survey of students in grades nine through 12, which gathered information from more than 115,000 adolescents throughout the United States. They analyzed information from 1999 to 2013. The survey has been conducted by the U.S. Centers for Disease Control and Prevention since 1990.

Use of other illegal drugs including hallucinogens and cocaine has also fallen throughout the time period, as has use of alcohol and cigarettes among teens. In 2013, 41.1 percent of the teens surveyed said they had smoked cigarettes in their lifetime, while 40.7 percent said they had used marijuana in their lifetime. The rates of cigarette smoking are down from 70.4 percent in 1999. Those who said they had ever tried alcohol fell from 81 percent in 1999 to 66.2 percent in 2013.

The findings are detailed in the journal Drug and Alcohol Dependence.