It's A Pain

Red haired people are more resistant to the pain-numbing effects of certain anesthetics. They require, on average, 19% more anesthetic. In 98% of the population the body produces a dark pigment eumelanin, but in redheads, a mutation in MC1R leads to the production of a red-tinged pheomelanin, instead. Pigment also interacts with molecules that are structurally similar to pigment stimulating hormones, including hormones whose primary role is one of pain relief.