Pain has a profound effect on our bodies - when we are experiencing it, millions of nerve cells deep within our brains are firing, telling us 'it hurts' - and for centuries the challenge has been to find something that will lessen or even switch off these sensations to bring us relief. Dr Michael Mosley discovers just what pain is, why we want to control it and how we ultimately did it when the discovery of morphine, the world's first pharmaceutical, at the beginning of the 19th century led to a 200-year journey of scientific breakthrough, discovery and self-experimentation.