The berry fruits of the capsicum are smaller and narrower than those of the sweet pepper and are somewhat bent with a pointed tip to the pod. As with chili pods, the distinctly hot flavor is due to the alkaloid capsaicin (C18H27NO3), which is primarily present in the partitions inside the pod and in the seeds. Most varieties contain capsaicin (methyl vanillyl nonenamide), a lipophile chemical that can produce a strong burning sensation in the mouth (and, several hours later, anus) of the unaccustomed eater. Most mammals find this unpleasant; however birds are unaffected, and it might appear that the fruit is "designed" for birds to spread the seeds. Chilli peppers are of great importance in Native American medicine, and capsaicin is used in modern Western medicine—mainly in topical preparations—as a circulatory stimulant and pain reliever.

The substances that give chiles their heat are the lipophile alkaloid capsaicin (8-methyl-N-vanillyl-6-nonenamide) and four related chemicals, collectively called capsaicinoids. Each capsaicinoid has a different effect on the mouth, and variation in the proportions of these chemical is responsible for the differing sensations produced by different varieties. Capsaicin causes pain and inflammation if consumed to excess, and can even burn the skin on contact in high concentrations (habaneros, for example, are routinely picked with gloves). It is also the primary ingredient in pepper spray, which is used as a defensive weapon. The "heat" of chiles is measured in Scoville units. Bell peppers rank at zero Scoville units, jalapeños at 3,000-6,000 Scoville units, and habaneros at 300,000 Scoville units. The record for the highest number of Scoville units in a chile is assigned by the Guinness Book of Records to the Red Savina Habanero, measuring 577,000 units! However, a recent report was made of a chile from India called the Naga Jolokia measuring at 855,000 Scoville units. Both the Red Savina and the Naga Jolokia claims are disputed as to their validity, and lack independent verification. The active ingredient in capsicum is called capsaicin which is derived from the same basic compound as the expectorant drug guaifenesin which is used in about 75 over-the-counter cough syrups and expectorants such as Robitussin and Actifed. This class of substance is known as mucokinetic, which means they move mucous along its route of exit from the body. Capsaicin is also classified as a diaphoretic, which means; t increases the release of toxins through sweating. So if you thought capsicum caused your sinuses to drain, your lungs to cough up phlegm and mucous while you seemed to sweat just a little more, medical science says you're right. So the next time that cold or flu strikes, you will know just what to do to relieve those congested lungs and stop build-up sinuses.