Dr Xand van Tulleken investigates the world of performance-enhancing drugs - from the athletes seeking the rewards of fame, glory and lucrative sponsorship deals to the hundreds of thousands of people in the UK now regularly taking anabolic steroids to look good and buff up. What are these drugs? What do they do to the body? And is it worth it? Xand's investigation reveals the extraordinary gamble dopers take with their health. Long-term effects include kidney failure, cognitive impairment and testicular shrinkage and Xand witnesses how users are self-experimenting with drugs that have not yet been approved for human use. Horizon uncovers the new frontier of doping, from new molecules to gene therapy - where the genes that control muscle growth are altered. These new methods could be completely undetectable by the doping authorities. Finally, and with the help of his twin brother Chris, Xand discovers the legal ways some athletes try to gain the edge.