Q: How many young people use cannabis? Cannabis, currently a class C drug, is the most commonly used illegal substance in the UK. Figures from the Department of Health show that 10 per cent of 11- to 15-year-olds reported using cannabis in 2006 and the separate British Crime Survey study, Drug Misuse Declared, found 20 per cent of 16- to 24-year-olds use the substance....