When part of your body is injured, special nerve endings send pain messages back to your brain. Painkilling drugs interfere with these messages, either at the site of the injury, in the spinal cord or in the brain itself. Many painkillers are based on one of two naturally occurring drugs: aspirin and opiates. Aspirin uses a chemical found in willow bark, used by the Ancient Greeks to relieve pain. Opiates all work in a similar way to opium, which is extracted from poppies.