Teens living in states that allow medical cannabis are less likely to use the drug compared to those in non-legal states, according to a new large-scale study.

The results, published in The American Journal of Drug and Alcohol Abuse, run contrary to the frequently expressed claims of marijuana legalization opponents who argue that loosening laws drives more youth to consume cannabis .

Researchers at the Boston College used the results of anonymous surveys given to more than 800,000 high school students across 45 states from 1999 to 2015 to calculate the number of teens who smoke cannabis. They investigated how self-reported marijuana use changed in states that have either decriminalized cannabis possession or legalized it for medical purposes.

The researchers found that the number of teenage cannabis smokers was 1.1% less in states that had enacted medical marijuana laws compared to those that hadn’t, even when accounting for other important variables such as tobacco and alcohol policies, economic trends, youth characteristics and state demographics.

“We found that for every group of 100 adolescents, one fewer will be a current user of marijuana following the enactment of medical marijuana laws,” says Dr Rebekah Levine Coley, a Professor of psychology at Boston College, who led the study.

“When we looked at particular subgroups of adolescents, this reduction became even more pronounced. For example 3.9% less Black and 2.7% less Hispanic youths now use marijuana in states with medical marijuana laws.”

The longer the medical marijuana laws have been in place, the greater the reduction in teen use

As the surveys were administered over a period of 16 years, the researchers were able to compare the changes in teenager’s marijuana use in states that adopted medical marijuana laws with those that have not. As a result, the study found that the longer the laws had been in place, the greater the reduction in teen marijuana use.

“Some people have argued that decriminalizing or legalizing medical marijuana could increase cannabis use amongst young people, either by making it easier for them to access, or by making it seem less harmful.” says Dr Rebekah Levine Coley.

“However, we saw the opposite effect. We were not able to determine why this is, but other research has suggested that after the enactment of medical marijuana laws, youths’ perceptions of the potential harm of marijuana use actually increased. Alternatively, another theory is that as marijuana laws are becoming more lenient, parents may be increasing their supervision of their children, or changing how they talk to them about drug use.”

Decriminalization has limited effect

The researchers also found that simply decriminalizing cannabis possession had no effect on teenage marijuana use, other than slight declines among 14-year-olds and Hispanic youth.

Neither medical marijuana or cannabis possession policies had any effect on frequent or heavy users of marijuana, suggesting that such students are not easily influenced by state laws.

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