Chillies contain a substance called Capsaicin. Capsaicin acts as an irritant on the lining of the eyes, mouth, nose, and digestive tract.
In small quantities, eating it may not be harmful. In fact, there are claims that Chillies may have some health benefits.
In large quantities, it will irritate the lining of the nose, eyes, mouth and skin. The immediate effect is watering of the eyes and burning sensation in the eyes, nose or mouth. It can cause ulcers of the mouth and stomach. If you already are suffering from ulcers, it can increase the severity.
It can also cause burning of the skin and extreme sweating. It can also cause raised heat rate and breathing. If you suffer from Haemorrhoids (piles), Chillies can cause increased bleeding.
As with everything in life, Chillies taken in moderation are enjoyable. In excess, they can cause many health problems.