“Pepper aficionados know that if one can withstand the initial sense that the mouth is on fire, desensitization follows. That is, the more you eat, the more you can tolerate. There is a physical basis for this phenomenon. In cells growing in culture and in laboratory animals, several hours of exposure to pure capsaicin leads to degeneration and death of the exposed tissues.
Researchers hypothesize that prolonged but not dangerous exposure to capsaicin by hot-food lovers may actually kill pain fibers, and this is why they can tolerate the spicy food. This effect has led to uses of capsaicin in topical treatments for arthritis and herpes outbreaks. The scientists’ elucidation of precisely how capsaicin sets the human mouth afire may lead to development of new types of drugs to treat chronic pain.”

Source: Google Answers