The latest data come from SAMHSA's 2015 National Survey on Drug Use and Health, a comprehensive survey of substance use among a nationally representative sample of 67,500 Americans age 12 and older. The trends in teen substance use are of particular interest to researchers, since initiation of drug and alcohol use at a young age is more closely linked to later health and behavioral problems than adult drug use.

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"These findings offer hope that marijuana and heroin use may be slowing down," said SAMHSA Principal Deputy Administrator Kana Enomoto in a statement. "And more American youth are rejecting alcohol and tobacco."

Nationally, marijuana use appears to be rising modestly among adults. In 2015, nearly 20 percent of 18 to 25 year olds used marijuana monthly, compared to about 17 percent in 2002. And monthly marijuana use among adults ages 26 years and higher rose from 4 percent to 6.5 percent over the same period. Rising marijuana use among the middle-aged and senior citizens are a big driver of that trend.

But at the same time, the prevalence of marijuana use disorders -- dependency on the drug, or abuse of it -- is falling. The incidence of marijuana use disorders among teens fell from 4.3 percent in 2002 to 2.6 percent in 2015. Among young adults, it fell from 6 percent to 5.1 percent. And among adults age 26 and older, marijuana use disorder has remained rare, hovering at around 0.8 percent.

Opponents of looser marijuana regulations have long warned that legalizing or decriminalizing marijuana would "send the wrong message" to adolescents and cause teen marijuana use to increase. "What about the children" remains a potent rallying cry for opponents hoping to stop legalization measures this year in California and elsewhere.