Capsaicin is the phytonutrient that makes chili peppers hot.

It stimulates the release of endorphins--hormones that block pain and enhance mood. It also helps to flush out the respiratory system by causing the body to release fluid that washes away mucus.

Capsaicin is used in an topical ointments to relieve arthritis and shingles pain. It relieves pain by "burning out" certain nerve cells, making them incapable of transmitting pain signals. Capsaicin is being studied for its effects on arthritis, cystitis, HIV and diabetic neuropathy.

References

Robbins W Clinical applications of capsaicinoids Clin J Pain 2000 Jun;16(2 Suppl):S86-9.