Published in the American Journal of Drug and Alcohol Abuse last month researchers found the number of teenage cannabis smokers was 1.1% less in states that had enacted medical marijuana laws (MML) compared to those that hadn’t.

The survey, based on data from more than 800,000 high school students across 45 states over 16 years, meant researchers could compare changes in teenager’s cannabis use in states that adopted MML with those that hadn’t.

Dr Rebekah Levine Coley, a professor of psychology at Boston College, who led the research, said: “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.

“Intriguingly, 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 decriminalising or legalising 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.

“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.”.