MONDAY, Nov. 6, 2017 (HealthDay News) -- Today's American teens are smoking less than ever, and the trend may be keeping many from smoking pot, too.
That's the finding of a new study that tracked more than 1 million teens from 1991 to 2016.
But the news wasn't all good, the researchers said. Kids who think marijuana is "safe" are more likely than their peers to use the drug. That's a concern because more and more, teenagers do believe pot is fairly harmless -- and experts say it's not.
The findings paint a nuanced picture.
It's known that U.S. teenagers' pot use has held fairly steady over the past decade -- even though kids have become increasingly more likely to believe the drug is harmless.
And that's puzzling, said Richard Miech, a research professor at the University of Michigan who led the new study.
There's good evidence, he explained, that when teens believe pot is safe, they're more likely to use it in the next year.
Miech and his colleagues wanted to figure out why pot use in teens isn't rising.
So they turned to a government-funded survey that has followed U.S. students since the 1970s.
Their conclusion: Teenagers today are not using pot in droves because they are much less likely to smoke cigarettes or drink alcohol than their predecessors were. Smoking, in particular, is a major factor in whether kids try marijuana, the researchers said.
For years, the study found, the percentage of teens who've ever smoked or tried alcohol has steadily dropped.
Cigarettes, in particular, have fallen out of favor. The percentage of kids who've smoked is now at "historic lows," Miech said.
In 2016, 28 percent of 12th graders said they'd ever smoked a cigarette. That was true of only 18 percent of 10th graders and 10 percent of kids in eighth grade.
And that seemed to explain why marijuana use has remained fairly steady since 2005 -- instead of skyrocketing as kids develop more friendly attitudes toward the drug.
"I think a big message of this study is that policies and interventions that reduce teen smoking seem to have the added benefit of reducing teen marijuana use," Miech said.
The findings appear online Nov. 6 in the journal Pediatrics.
Then there's the not-so-good news: Over the past decade, the percentage of 12th graders who've used marijuana in the past year has stubbornly hovered over 30 percent.
And based on Miech's findings, kids who believe pot is harmless are still more likely to use it.
That means education about the harms of marijuana is still critical, said Dr. Nicholas Chadi, a pediatrician and adolescent medicine specialist at Boston Children's Hospital.
"Marijuana is harmful to the developing adolescent brain," said Chadi, co-author of an editorial published with the study. Plus, he added, teenagers who use the drug are more prone to trying other illegal substances.
The findings are relevant to current debates over legalizing marijuana, Miech and Chadi said.
Eight U.S. states and Washington, D.C., have legalized marijuana for recreational use. Many more states allow medical use.
Some argue that legalizing pot could lead some kids to believe it's safe. That, in turn, could make them more likely to use it.
But, Chadi said, "it's not that legalization is good or bad."
Instead, kids need to learn about the risks of marijuana, he said.
Miech agreed. Tobacco and alcohol are both legal for adults, but kids are warned about their dangers, he noted. "We've seen tremendous declines in adolescent use of both substances over the past two decades, so we have examples to work from," he said.
It's important for parents to let their kids know they do not approve of any drug use, Miech said. "When we ask, about 90 percent of adolescents say their parents would disapprove of their marijuana use," he said.
"When this message is reinforced in schools and the media," Miech said, "we've been able to see progress in reducing youth drug use."
The U.S. Centers for Disease Control and Prevention has more on teens and marijuana use.