According to a large-scale study of American high school students, legalising medical cannabis has actually led to a drop in its use among teenagers.

The study, published in the American Journal of Drug and Alcohol Abuse, used the results of an anonymous survey given to more than 800,000 high school students across 45 states to calculate the number of teens who smoke cannabis.

In states that have enacted medical cannabis laws, the study found that the number of teenage cannabis users was 1.1% less that in states that have not. This result was found even when taking into account other variable factors such as economic trends, state demographics and youth characteristics, as well as tobacco and alcohol policies.

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

“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 cannabis laws”.

This particular survey took place over a period of 16 years – allowing the researchers to compare changes in teenage use of cannabis in states that have enacted medical cannabis laws. Results from the study showed that the longer the laws had been in place the less the use of cannabis was amongst teenagers.

Dr Coley said: “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 cannabis laws, youths’ perceptions of the potential harm of cannabis use actually increased.

“Alternatively, another theory is that as cannabis laws are becoming more lenient, parents may be increasing their supervision of their children, or changing how they talk to them about drug use”.

One of the prominent findings of the study was that legalising recreational marijuana had no noticeable effect on adolescents’ cannabis use, except for a small decline in marijuana smoking among 14-year-olds and people from Hispanic backgrounds, and an increase in use among white adolescents.

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