Does legalization of marijuana for adults lead to greater use among teenagers?

The long-awaited results of the first major analysis are in. And the answer is: Yes. And no.

The UC-Davis and Columbia University study, published Tuesday in the journal JAMA Pediatrics, compares before-and-after perceptions of marijuana and its use among youth in Colorado and Washington. (Research was concluded before last November’s passage of Prop. 64, legalizing adult recreational cannabis in California.)

It found worrisome increases in marijuana use among younger adolescents in Washington.

But it found no increase in use in younger adolescents in Colorado.

Nor was there any change in use among older adolescents in either Washington or Colorado.

“It is fair to say that we are still uncertain how to explain the results,” wrote one of the nation’s leading pediatricians, Dr. Alain Joffe of the Johns Hopkins University School of Medicine, in an accompanying editorial.

The United States is undergoing a profound shift in the legal status of marijuana, with the percentage of Americans living in states where marijuana use is legal for adults now above 20 percent. Eight states and Washington, D.C. have legalized the use of small amounts for recreational use, 21 states decriminalizing possession of small amounts of marijuana for personal use and 25 states and Washington, D.C. have enacted medical marijuana laws.

There is growing concern that legalization for adults will lead to greater use among youth. Young people are at greatest risk of adverse effects, especially if they begin use in their teens and use daily throughout young adult life.

“The industry will protest that their promotions are intended only for adults, but adolescents cannot be insulated from their messages,”  wrote Wayne Hall of the University of Queensland in Australia, in another accompanying editorial.

Previous research by the nationwide Monitoring the Future study, funded by the National Institute on Drug Abuse, found that  daily use of marijuana by adolescents has increased,  from 5 percent in 2006 to 5.8 percent and 6 percent in 2014 and 2015, respectively, among 12th graders.

The new JAMA Pediatrics research, lead by Dr. Magdalena Cerdá, associate professor in emergency medicine and associate director of the UC-Davis Violence Prevention Research Program, had split findings.

  • In Washington, legalization of recreational marijuana use significantly reduced perceptions of marijuana’s harmfulness by 14 percent and 16 percent among younger adolescents (8th and 10th graders) and increased these students’  past-month marijuana use by 2 percent and 4 percent  —  but such shifts were not seen in Colorado’s younger adolescents.
  • Among older adolescents (10th graders) in Washington and Colorado, there were no changes in perceived harmfulness or marijuana use in the month after legalization.
  • In 45 states without legalized marijuana use, perceived harmfulness marijuana also decreased by 5 percent and 7 percent for younger adolescents (8th and 10 graders.) But marijuana use declined, as well,  by 1.3 percent and .9 percent.

Confused? If legalization caused changes among youth in one state, we might expect a similar trends among youth in another state.

But researchers offer this possible explanation: Perhaps Colorado teens were already familiar with marijuana, even before recreational legalization, because that state’s medical marijuana laws were much more liberal than those in Washington. Unlike Washington, Colorado already had a well-developed medical marijuana dispensary system and substantial advertising for its medical pot stores.  So, for teens, maybe legalization didn’t change much.

Also puzzling is why legalization changed attitudes and use among younger students in Washington — but not older students. Perhaps, the scientists say, older students already had well-formed attitudes. So legalization didn’t alter their opinions.

As usage becomes more accepted among youth, here’s another unknown: Will more teens use alcohol and marijuana together — or will marijuana use replace alcohol use?

If they’re combined, that’s bad news, because it could increase impairment while driving, wrote Joffe.

But if marijuana is substituted for alcohol, there could be a reduction in aggressive behaviors such as assault, sexual assault or date rape, he said.

The inconclusive new study shows the need for greater research and prevention strategies to limit harm, he said.

“How best to address this new reality?” asked Joffe. “What lessons can we draw from what we know about curtailing use of alcohol and tobacco among teenagers?”

“These are but a few of the questions,” he wrote, “that will need to be addressed in the coming years.”