In 1982, then-First Lady Nancy Reagan launched a now-famous campaign urging teens to "Just Say No" to marijuana and other drugs. Today, with more states legalizing weed for medicinal and recreational purposes, some health professionals -- concerned about its lasting impacts on the developing teen brain -- send a different message: "Just not yet."
"There is a growing assumption among teens that if marijuana is legal for adults, it can't be bad for you. We know that's not necessarily true. When you start using it matters," says Staci Gruber, an associate professor of psychiatry at Harvard Medical School and director of the Marijuana Investigations for Neuroscientific Discovery (MIND) lab. " 'Just Not Yet' conveys the message that you should give your brain a chance to be more fully developed before you expose it to something that could change its trajectory."
More Acceptance, More Concern
Teens are more inclined to try pot today than they have been in decades, with 24% of 8th, 10th, and 12th graders already using it and one in four high school seniors saying they would try it or use it more if it were legal in their state, according to a 2018 national survey.
More parents are also using pot around their kids, either recreationally or medicinally, and perceptions of risk among adults and youths have been declining for years, research shows. That concerns Seth Ammerman, MD, a clinical professor in the division of adolescent medicine at Stanford University. He says that while cannabis can be relatively safe and therapeutic for adults when used responsibly, its hazards -- particularly for young people -- are real.
"What is benign for the parent is not necessarily benign for the adolescent," he says.
The brain is still taking shape well into the 20s, with the prefrontal cortex -- the region involved in decision-making, planning, problem-solving, and controlling impulses -- developing last. Using marijuana during this vulnerable time can affect that development, research suggests.
For instance, brain imaging studies show that people who started using marijuana regularly before age 16 have less developed white matter, the nerve fibers that transmit messages from one area of the brain to another. One recent animal study showed that exposure to the equivalent of about a joint a day during adolescence can stunt the growth of key brain circuits that affect the ability to make decisions later on.
"It appears to alter how the brain is wired, and then you are stuck with it," says lead study author Jamie Roitman, an associate professor of psychology at the University of Illinois at Chicago. People who start using marijuana frequently as teens also behave more impulsively and tend to score worse on cognitive tests than non-users or those who started using marijuana later. A few small studies have shown they are more prone to depression and psychosis.
Not Your Parents' Weed
Ammerman says the marijuana that teens can use today is far more potent than what was around a generation ago. With the advent of edibles and marijuana concentrates like "shatter" and "butter," potency can get even higher, likely boosting the risk of bad effects.
He says he is often asked by parents and teens to prescribe medicinal marijuana for adolescents. After weighing benefits vs. risks, he typically doesn't recommend it. "While it has been shown to be helpful for adults in addressing some symptoms, there is virtually no research on its use in the adolescent population yet, so appropriate dosing, effects, and side effects are still unknown."
What should parents -- even those who use marijuana themselves -- tell their kids? Wait. "Messages of 'Just Say No' or complete abstinence don't work. We know that from history," says Gruber. "We are just saying hold off. It's a better long-term strategy for your brain."
By the Numbers
10: Number of states, along with the District of Columbia, that have legalized recreational marijuana. Medicinal marijuana is legal in 33 states.
12%: The average percentage of tetrahydrocannabinol (THC) in marijuana today. In 1980, it hovered around 4%.
9%: Percentage of people who experiment with marijuana who become dependent. This rises to 17% among those who start using it as teens, and 25% to 50% for teens who smoke daily.
37%: Percentage of high school seniors who use marijuana. So do 26% of 10th graders and 10% of 8th graders.
25: Age at which the brain is mostly done developing.
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