Capsaicin is the chemical compound that makes chili peppers taste hot. It is a skin irritant to humans and animals, and can cause a burning sensation if applied to the skin.
Ingredients containing high amounts of capsaicin are widely used as flavorings in spicy cuisines. All members of the pepper family that are hot have high levels of this compound. Hot sauces such as hot salsa and Tabasco are high in it, and people can easily burn their mouths eating spicy foods if not careful.
Drinking water to quell the heat caused by too much capsaicin in a meal is ineffective, since the compound is not water-soluble. The most effective antidotes are those containing fat and/or sugar. Capsaicin is fat-soluble, and sugar will block the receptors that are reacting with it, so if someone is really suffering, go for cheese or ice cream instead of ice water or beer. The compound will bond with the fat in the food end lessen the pain.
Capsaicin is most recently making its heat felt in topical analgesics. Sports creams and muscle rubs that seem to heat the skin often now contain the compound either in lieu of, or in addition to, the more traditional menthol. Some say that it is showing some promise in treating arthritis symptoms; it seems over time to have a cumulative effect on the pain, by overloading the nerves and lessening their ability to transmit pain signals to the brain. Many of the new patches that adhere to the skin and are placed over the painful area are now using capsaicin to effect their promised pain relief.
It is also making news in the diet and nutritional fields. Some studies show that a diet high in chilies or other capsaicin-laden foods can raise a person's base metabolism, the rate at which the body burns fat while at rest. There are also studies claiming it both lowers LDL, or "bad cholesterol," and raises HDL, or "good cholesterols" too.
With all that going for it, people may want to consider themselves lucky if they like spicy cuisines, and help themselves to as much heat as their mouths can stand.