Two point four million people smoke cannabis in the UK. Some police forces don't prosecute for possession any more, and doctors can now apply for licences to prescribe the cannabis to treat patients with certain medical conditions. Canada has just legalised it, as have several American States. So is it time to look at the evidence and assess whether UK policy needs to change? Former Met Police chief Lord Hogan-Howe, who has always supported tough laws on cannabis, investigates for Dispatches. He visits Colorado - one of the first states in the US to legalise - and sees first-hand how the cannabis business is booming, how the state is using its quarter-of-a-billion-dollar tax dividend and discovers that some strains of cannabis on sale are six times stronger than skunk. Will he change his mind about legalisation?