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Teenagers are turning their back on marijuana in a U.S. state where the drug has been legalised because it is no longer 'cool'.

The legalisation of cannabis in Colorado led to the drop with one theory suggesting it's an 'adult' drug.

Kayvan Khalatbari, co-founder of a marijuana dispensary, said: "Cannabis, now that it's legal, kind of is an old person's drug.

"It is something that kids are seeing adults use all over the place. It just doesn't seem as cool to kids anymore."

However, the Washington Examiner says that public officials are saying that the drop is as a result of an anti-cannabis campaign.

The use of the drug on a recreational basis was legalised in 2012 and the Colorado Department of Public Health and Environment said youngsters found the drug less risky.

In 2008, 58 per cent thought it was risky compared with 54 per cent in 2013.

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Smoking tobacco is also down, at a faster rate than the drop in cannabis, which is put down to a combination of smoking bans, tobacco taxes, enforcement and awareness campaigns.

Larry Wok, executive director and chief medical officer, said: "We know what works to protect young people from unhealthy substances.

"As with tobacco, youth prevention campaigns will help ensure adult legalisation of mairjana in Colorado does not impact the health of Colorado kids.

"If we want Colorado to be the healthiest state in the nation, then we need to make sure our youngest citizens understand the risks of using potentially harmful substances," Dr Wolk added.

"Later this month, we'll launch a youth prevention campaign that encourages kids not to risk damaging their growing brains by experimenting with marijuana."