(Reuters Health) - When medical marijuana becomes legal in a state, teenagers there may be slightly less likely to use the drug, a U.S. study suggests.

Even though medical and recreational marijuana laws restrict use to adults, access for adults may influence how easily teens can get the drug and whether they use it, researchers write in the American Journal of Drug and Alcohol Abuse.

To see how teen marijuana use compares in states with and without such laws, researchers examined survey data on substance use collected from 861,082 adolescents in 45 states, aged 14 to 18 years between 1999 and 2015. Among the states included in the study, 11 had legalized recreational marijuana and 18 had legalized medical marijuana by April 2015.

“Adjusting for numerous other substance use policies, medical marijuana laws were associated with small declines in current marijuana use among adolescents, with larger declines in some subgroups, such as males and African American and Hispanic adolescents,” lead study author Rebekah Levine Coley, a psychology researcher at Boston College, said by email.

Researchers found that teen marijuana use varied widely by state, from an average of 8.6 percent in Utah to nearly 28 percent in Vermont. To adjust for these interstate differences, they used a statistical model and actual reported usage rates by teens in each state over the study period to calculate the probability, or risk, of teen marijuana use. Overall, this probability was 18.9 percent in states with laws permitting medical marijuana, and 20 percent in other states, the study found.

This difference of just over 1 percentage point was more pronounced among certain groups of teens, however.

The probability of black youth using marijuana, for example, was 3.9 percentage points lower in states with medical marijuana laws than in other states, while it was 2.7 percentage points lower for Hispanic youth.

Teen marijuana use also declined more with each year that their state had a medical marijuana law on the books, the analysis found.

Recreational marijuana laws didn’t appear to influence whether teens used pot, except for a slight reduction in use by 14-year-olds and Hispanic youth.

Neither recreational nor medical marijuana laws appeared to impact whether adolescents became heavy pot users.

State increases in beer and cigarette taxes also didn’t appear to impact whether teens used marijuana. Some previous research suggests that when one type of substance gets more expensive, teens turn to an alternative.

It’s possible that some teens perceived marijuana as more harmful once states passed medical marijuana laws because they started thinking of it like a prescription medicine with potential side effects, the researchers note.

One limitation of the study is that it relied on surveys done in high school, leaving out youth who drop out and are also at higher risk for drug use.

Researchers also didn’t look at health outcomes of marijuana use in teens.

Other studies have linked marijuana to a range of behavioral and emotional problems in teens. Some previous research also suggests that effects of marijuana use may be more pronounced in teens than adults because adolescence is a period of rapid brain development.

“Marijuana use can interfere with the frontal cortex, the region of the brain that handles executive function and isn’t fully developed in adolescence, as well as the brain’s limbic system, which handles impulse control,” said Dr. Ellen Rome, head of the Center for Adolescent Medicine at Cleveland Clinic Children’s Hospital in Ohio.

The result can be loss of memory, impaired motor coordination, and clouded or impaired judgment, Rome, who wasn’t involved in the study, said by email.

“The adolescent may think they are functioning just fine but not remember as well on a test or not realize that they have delayed reaction time when driving or trying to do something physical,” Rome said.

“Heavy or prolonged use can lead to higher anxiety rates, cyclic vomiting, lower educational achievement, and apathy or disengagement from all activities that do not support use,” Rome added. “In other words, kids may use marijuana to handle stress or anxiety, but this strategy backfires over time.”

SOURCE: bit.ly/2XfTo3m The American Journal of Drug and Alcohol Abuse, online February 14, 2019.