Marijuana use among U.S. teens has risen since the early 1990s, even as alcohol and tobacco use has fallen.

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Experts say parents should discuss marijuana and its health impacts with children as early as fifth grade. Getty Images

Teenagers in the United States are using less alcohol and tobacco today than they did in the early 1990s, but marijuana use has risen sharply during that time, a new study shows.

This comes at a time when 33 states and the District of Columbia have legalized medical or recreational marijuana, or both.

The study doesn’t show how marijuana use has changed in states that legalized recreational marijuana.

But the overall trend in the country is toward greater use of marijuana among high school students.

“The findings clearly show that more youth are using marijuana. So education by parents and schools is certainly necessary,” Bonnie Halpern-Felsher, PhD, a professor of pediatrics at Stanford University, told Healthline.

The new study, published in the American Journal of Public Health, looked at the use of alcohol, cigarettes, and marijuana by U.S. high school students over the past quarter century.

Between 1991 and 2017, marijuana-only use increased from less than 1 percent to slightly more than 6 percent.

During this time, both alcohol-only and cigarette-only use decreased — down to 12 percent and 1 percent, respectively.

Researchers also found that the percentage of students who reported using both alcohol and marijuana more than doubled, while dual-use of alcohol and cigarettes decreased significantly.

Marijuana-only use increased across all races and ethnicities, with the largest increases among African American and Hispanic teens.

The data comes from the national Youth Risk Behavior Survey, which is taken every two years of U.S. students in grades 9 through 12. The study focused on “current” use, which is defined as use during the past 30 days.

With the rise in marijuana use among teens, parents now have another risky behavior to talk to their children about.

Dr. Amy Sass, an adolescent medicine specialist at Children’s Hospital Colorado, said parents should take some time to educate themselves about marijuana and related products such as cannabis oil and marijuana edibles.

This includes knowing your state’s marijuana laws, which vary across the country. One easy thing to remember, though, is that recreational use of marijuana by those under the age of 21 years is illegal in all states.

Parents should also learn about how marijuana and THC — the main active ingredient of cannabis — affect the body and brain.

And they should know that marijuana is addictive.

“In adolescents, the brain is still developing, so when we introduce a drug into the brain — particularly nicotine or THC — they’re more likely to become addicted,” said Halpern-Felsher. “It actually changes their brain chemistry to want more of the drug.”

Young people who regularly use marijuana are also more likely to have other mental health problems, including anxiety, depression, and paranoia. Marijuana can even make existing mental health problems worse.

Overdosing on marijuana is also a real possibility, especially for new users.

This is particularly true with marijuana edibles such as brownies, candies, and other foods. It takes much longer to feel an effect from edibles — two to three hours — compared to smoking marijuana.

“Young people sometimes take a small square of a brownie, but when they don’t feel an effect right away, they take another piece and another piece,” said Halpern-Felsher. “Suddenly, half an hour later they’re quite sick and wind up in the hospital.”

Sass told Healthline that children are exposed to messages about marijuana even before high school, through social media, friends, billboards, and store marketing.

Parents should start having conversations about marijuana when their children are younger, even as early as fifth or sixth grade.

“It’s actually good for parents to bring it up,” said Sass. “Parents know their kids the best. If they have a feeling that they should be discussing these topics, then they really should just go for it.”

Children may also be learning about marijuana through their school, after-school groups, or other programs. But parents shouldn’t rely on these to get their kids talking.

“It’s critically important to also have these conversations at home,” said Sass.

Talking about marijuana with your kids is also not a one-time thing.

“You want to keep checking in with teens about it,” said Sass, “particularly as they’re gaining their independence and spending more time with their friends versus family.”

Halpern-Felsher said when parents talk to children about the health risks of marijuana, they should have an honest conversation, saying something like: “We’d rather you don’t use marijuana because there are some dangers to it. But if you do use it, the better choice is to delay as long as you can so you don’t have a lifetime addiction potential.”

Sass suggests setting clear family expectations about marijuana use and safety — including telling teens to never operate a vehicle if they are impaired by drugs or alcohol, and never riding in a car with a driver who is impaired by drugs or alcohol.

But parents should be careful not to shut down the conversation by judging too quickly the actions of their child or friends.

“Talking to teens about marijuana use and other risky behaviors is really more about listening than talking,” said Sass.