Teenagers smoke less weed in US states where medical marijuana is legal, a new study finds.

Researchers said that for every 100 teens, one fewer used pot in state that allows adults to buy and use cannabis for medical purposes, recreation, or both.

The effect was more obvious in minority teens, with nearly seven percent fewer black and Hispanic teens likely to smoke a joint in states where doing so is legal for adults than in states where it isn't.

Previous studies have looked at teenage use in individual states, and in states with recreational marijuana laws.

But the team, led by Boston College in Massachusetts, says its study is one of the first to look at the association between medical marijuana legalization and teen use.

A new study has found that there are 1.1 percent fewer teenage weed smokers in states with medical marijuana laws than in states without them (file image)

Medical marijuana is currently legal in 33 states and the District of Columbia.

Ten of those states - Alaska, California, Colorado, Maine, Massachusetts, Michigan, Nevada, Oregon, Vermont and Washington - and DC have also legalized recreational use.

Critics have bemoaned that legalizing marijuana in any form will pose a significant public health threat- especially to children and teenagers - while proponents say teens will be less likely to abuse a drug they can easily get their hands on.

For the study, published in the American Journal of Drug and Alcohol Abuse, the team surveyed more than 800,000 high school students across 45 states over 16 years.

Results showed there are 1.1 percent fewer teenage smokers in states with medical marijuana laws than in states without them.

'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,' said lead author Dr Rebekah Levine Coley, a professor of psychology at Boston College.

Dr Coley said the difference was even more 'pronounced' when they looked at different racial and ethnic groups.

They found that there were nearly four percent fewer black teens and almost three percent fewer Hispanic teens smoking pot in states with medical marijuana laws.

In 2003, when surveys first began being administered, only 10 states had legalized medical marijuana.

But the researchers said this is made easier to compare teen cannabis use in a state before and after medical marijuana was legalized.

Survey results also found that the longer the law has been in effect in a state, the fewer teen smokers there were.

The researchers say they found that legalizing recreational marijuana had no effect on teen smoking aside from an increased use in white teens.

Dr Coley says the findings offer a new prospective on the debate about the benefits and risks of US marijuana legalization and decriminalization.

'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,' she said.

'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.'