Even with more U.S. states legalizing marijuana, use of the drug among American teenagers has declined -- as has alcohol consumption -- in 2014, a study found.

That's the finding of the Monitoring the Future study, conducted by the University of Michigan and going into its 40th year, which polled around 40,000 8th, 10th and 12th grade students in schools across the country on their use of cigarettes, alcohol and drugs both legal and illegal.

Less than 15 percent of 12th grade student said they smoked cigarettes, down from over 35 percent in the late 1990s; alcohol consumption by that same group was down from 55 percent seen in 1992 to below 40 percent; and marijuana use, after 5 years of increases, dropped slightly from 26 percent in 2013 to 24 percent in 2014.

That's despite Colorado and Washington state joining a number of other states in making marijuana legal for adults.

"So far, that doesn't seem to have made teens more likely to use marijuana," the National Institute on Drug Abuse said in a blog.

Still, there's work still to be done when it comes to teens and drug use, says study principle investigator Lloyd Johnston.

"There is a lot of good news in this year's results, but the problems of teen substance use and abuse are still far from going away," he says.

Although use of traditional tobacco products is down among teens, the study results suggests many teens may be trading traditional cigarettes for e-cigarettes, as more teens report using e-cigarettes that conventional cigarettes or another other tobacco products.

When it came to abuse of prescription drugs, including narcotic painkillers, amphetamines and sedatives, that dropped from 16 percent in 2013 to 14 percent this year among 12th graders, the study found.

Because narcotic pain relievers such as Vicodin and OxyContin are prescribed by doctors, some teens have considered them safer than illegal drugs such as cocaine and heroin, an issue that is being addressed, experts say.

"There's a very strong and aggressive campaign about educating the public on the risk of opioid medications as it relates to overdoses and deaths," says Nora Volkow, director of the National Institute on Drug Abuse. "That has made teenagers aware that they are not so safe as they thought they were."

"Even though the [study's overall] indicators are very good news, at the same time we cannot become complacent," she says. "This is a stage where their brains are most vulnerable. We need to continue our prevention efforts."

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