Professor David Nutt was sacked in 2009 as the government's chief drugs adviser after criticising its decision to reclassify cannabis. He is a psychiatrist and one of the country's leading experts on the effects of drugs on the brain. His latest research is investigating how psilocybin, the active ingredient in magic mushrooms, could be used to treat depression. He talks to Jim about his passion for science and his disputes with government over drug policy.