Teenagers are less likely to use cannabis in US states where the drug has been legalised, a new study suggests.

Researchers at Montana State University in Bozeman, Montana, analysed annual national Youth Risk Behaviour Surveys from 1993 through to 2017.

The reports included questions about marijuana use and listed responses from 1.4m high school students.

During that time period, 27 states legalised medical marijuana while seven states legalised cannabis for recreational purposes.

Today, some 33 states have legalised medical cannabis, while 10 states have also legalised recreational use.

Where is cannabis legal in the US? Show all 11 1 /11 Where is cannabis legal in the US? Where is cannabis legal in the US? Alaska Recreational cannabis use was made legal in Alaska in 2014 Getty Where is cannabis legal in the US? Washington Washington was among the first states to legalise cannabis in 2012 Getty Where is cannabis legal in the US? Michigan Recreational cannabis use was made legal in Michigan in 2018 Getty Where is cannabis legal in the US? Massachusetts Recreational cannabis use was made legal in Massachusetts in 2016 Getty Where is cannabis legal in the US? Colorado Colorado was among the first states to legalise cannabis in 2012 Getty Where is cannabis legal in the US? Vermont Since January 2018 it has been legal to posses up to one ounce and two cannabis plants in Vermont. Commercial sales are not legal Getty Where is cannabis legal in the US? California Recreational cannabis use was made legal in California in 2018 Getty Where is cannabis legal in the US? Illinois Recreational cannabis use was made legal in Illinois in 2019 Getty Images Where is cannabis legal in the US? Oregon It was made legal in Oregon to possess up to one ounce of cannabis in 2014 Getty Where is cannabis legal in the US? Maine Recreational cannabis use was made legal in Maine in 2016 Getty Where is cannabis legal in the US? Nevada Recreational cannabis use was made legal in Nevada in 2016 Getty

The drug remains illegal in all states for people under the age of 18.

After examining the reports, the researchers found that there was an eight per cent drop in the number of teenagers who said they used the drug at least once in the last 30 days.

Meanwhile, the number of youths who had used cannabis at least 10 times in the last 30 days saw a nine per cent drop.

While the findings suggest that legalising recreational marijuana is not tied to an increase in drug use, lead author of the study Mark Anderson has suggested that the drop could be because it is usually harder for teens to buy from licensed dispensaries where proof of age is required.

Anderson added that he hopes the study, which was published in the medical journal JAMA Pediatrics on Monday, will "help to quell some concerns that use among teens will actually go up" in places where recreational marijuana is legalised.

”Because many recreational marijuana laws have been passed so recently, we do observe limited post-treatment data for some of these states,“ Anderson told CNN.

”In a few years, it would make sense to update our estimates as more data become available.“

The results echo those of a previous study by the RAND Corporation - an American nonprofit global policy think tank - which found that cannabis use among teens in Washington dropped after the state legalised the drug in 2012.

Use among students in the eighth grade dropped from 9.8 to 7.3 per cent between 2014-2016, while use among 10th graders fell from 19.8 to 17.8 per cent.

However, the results do contradict the 2017 Healthy Kids Colorado Survey conducted by the Colorado Department of Public Health and Environment.

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The poll of 56,000 youth found that the number of high school pupils who said they used cannabis stayed the same after recreational use was legalised in that state in 2014.