By Pat Anson, Editor

A large survey of nearly a quarter of a million adolescents indicates the number of American teenagers with marijuana related problems is declining – despite the fact that nearly half the states have legalized medical marijuana or decriminalized it.

Researchers at Washington University School of Medicine in St. Louis studied a national database on drug use by over 216,000 young people, ages 12 to 17, and found that the number dependent on marijuana or having trouble in school and in relationships declined by 24 percent from 2002 to 2013.

During the same period, the number of kids who said they used marijuana in the previous 12 months fell by 10 percent. The drops were accompanied by reductions in behavioral problems, such as fighting, shoplifting and selling drugs.

Researchers believe the two trends are connected -- as kids became less likely to engage in problem behavior, they are also less likely to have problems with marijuana.

"We were surprised to see substantial declines in marijuana use and abuse," said lead author Richard Grucza, PhD, an associate professor of psychiatry at Washington University School of Medicine

"We don't know how legalization is affecting young marijuana users, but it could be that many kids with behavioral problems are more likely to get treatment earlier in childhood, making them less likely to turn to pot during adolescence. But whatever is happening with these behavioral issues, it seems to be outweighing any effects of marijuana decriminalization."

The new study is published in the Journal of the American Academy of Child & Adolescent Psychiatry. The data was gathered as part on ongoing study called the National Survey on Drug Use and Health, which surveys young people in all 50 states about their drug use, abuse and dependence.

In 2002, just over 16% reported using marijuana during the previous year. That number fell to below 14% by 2013. Meanwhile, the percentage of young people with marijuana-use disorders declined from around 4% to about 3%.

"Other research shows that psychiatric disorders earlier in childhood are strong predictors of marijuana use later on," Grucza said. "So it's likely that if these disruptive behaviors are recognized earlier in life, we may be able to deliver therapies that will help prevent marijuana problems -- and possibly problems with alcohol and other drugs, too."

A similar survey, the University of Michigan's Monitoring the Future Study, found that marijuana use by teens has leveled off since 2010, but was still at stubbornly high rates. In 2015, about 35% of 12th­ graders reported using marijuana at least once in the past year.

The same survey found that teenage abuse of prescription opioids declined for the fifth year in a row. Only about 5% of 12th graders reported using an opioid pain medication in the last year, and the number reporting that prescription opioids were “fairly easy” or “very easy” to get also continues to drop.

Medical marijuana is legal or decriminalized in 24 states and the District of Columbia, and several states are considering legalization. Opponents have long maintained that legalization would have harmful effects on young people.

“Perhaps the biggest public health concern around medical marijuana liberalization and legalization concerns the potential impact on teenagers, who could have greater access to it as a drug of abuse and who may increasingly see marijuana as a ‘safe, natural’ medicine rather than a harmful intoxicant,” wrote Nora Volkow, MD, director of the National Institute on Drug Abuse, in Alcoholism & Drug Abuse Weekly.

“Although there is still much to learn about marijuana’s impact on the developing brain, the existing science paints a picture of lasting adverse consequences when the drug is used heavily prior to the completion of brain maturation in young adulthood. In teens, marijuana appears to impair cognitive development, may lower IQ and may precipitate psychosis in individuals with a genetic vulnerability.”

According to a recent report from the Colorado Department of Public Safety, where marijuana has been fully legalized since 2013, nearly a third (31%) of young adults, ages 18 to 25, have used in marijuana in the last 30 days, up from 21% in 2006. The number of juveniles on probation testing positive for THC has also increased since legalization.