In November last year the American state of Colorado voted to legalise the recreational use of cannabis. It is the most radical experiment in drugs policy for generations and the world will be looking to see what happens, particularly to drug use amongst teenagers. In this hour long documentary for This World, clinical psychologist and addiction expert Professor John Marsden heads to Denver, the state capital, to assess the likely impact of legalisation on a country already suffering an epidemic of teenage marijuana use.

At a local high school, John hears from A grade students who explain that getting stoned is now more socially acceptable than getting drunk. At an addiction clinic that treats children as young as 12, John hears how marijuana is already the number one reason for kids to enter residential programmes more than alcohol, cocaine, ecstasy and other drugs combined. With rates of teenage cannabis use in the USA the highest that they have been in years, it is widely acknowledged the war on drugs has failed. However the question is, will full legalisation manage to take the selling of the drug out of the hands of the street dealers and into the hands of the legitimate business people and be the answer to stopping America's kids from getting stoned?