Could the power of fake pills be used to treat some of our most common medical complaints? To find out, Dr Michael Mosley embarks on Britain's largest-ever trial to investigate the placebo effect. He heads to Blackpool to gather 117 people suffering from backache - one of the leading causes of chronic pain. The volunteers believe they taking part in a trial for a powerful new painkiller - but Michael is treating them with nothing but fake pills and the power of the mind.