The heat in chilli peppers comes from capsaicin, a potent compound concentrated mainly in the white ribs and seeds, but also distributed unevenly through the flesh, to which it gives a distinctive tongue-tingling flavour.
When eaten in spicy dishes, capsaicin produces a burning sensation in the mouth, making the eyes water and the nose run. In some cases this may help to clear blocked airways by thinning down the mucus in the sinuses.
Chillies are richer in vitamin C than citrus fruits, but are unlikely to contribute much to the daily intake as they are usually eaten only in small amounts. However, some milder varieties can be eaten in larger quantities.
Some researchers have claimed that eating chillies may cause the stomach to secrete a mucus which protects the lining against irritants such as acid, aspirin or alcohol. However, the chillies themselves can sometimes irritate the digestive system and cause extensive itching in the anal area. Chillies can also act as anticoagulants and may help to lower blood pressure and blood cholesterol levels.