Capsaicin is the main chemical that makes chili peppers hot. Capsaicin is an animal repellent that is also used against insects and mites. Capsaicin was first registered for use in the United States in 1962. The U.S. Environmental Protection Agency (U.S. EPA) considers it to be a biochemical pesticide because it is a naturally occurring substance.

Products containing capsaicin can be aerosols, liquids, or granular formulations. There are several dozen products containing capsaicin on the market in the United States. Many of these products are animal repellents. Always follow label instructions and take steps to avoid exposure.

Capsaicin is very irritating to the skin and eyes, and it causes swelling in lung tissue. It can also irritate the mucous membranes in the mouth. In insects and mites, it appears to damage membranes in cells and disrupt the nervous system.

Products containing capsaicin are used to deter bears and dogs. They may also be used as repellents against wildlife or pets in backyards and gardens. Other products can be used in nurseries or in agriculture. You may be exposed if you are applying capsaicin and you breathe it in or get it on your skin. You may also be exposed if you eat or smoke after using a product that contains capsaicin without first washing your hands. You also may be exposed if you touch plants that are still wet with spray. You can limit your exposure by staying away from plants that have been sprayed with capsaicin until they are dry. You can also limit exposure by following all label instructions carefully.

Humans do not absorb capsaicin through their skin. In animals, researchers have shown that capsaicin is absorbed by the stomach. Capsaicin moves throughout the body with the greatest amounts in the brain and spinal cord. The liver breaks down capsaicin. In one study, researchers could not find any residues of capsaicin 17 hours after injecting rats with it.

Whether capsaicin causes cancer in animals is uncertain. The U.S EPA has not published a cancer rating for capsaicin. One study found that people eating 90-250 milligrams of Jalapeño peppers were more likely to develop gastric cancer than people eating less than 30 milligrams of Jalapeño peppers a day. Another study found that pure capsaicin seemed to stop human prostate cancer cells from increasing in number.

Scientists studied developmental and reproductive effects of capsaicin in rabbits and rats. The bones in the paws of baby rats did not develop normally when their pregnant mothers were fed capsaicin at doses that made the mothers sick. Baby rabbits were not affected even when their mothers ate enough capsaicin to be sick. There was no information about reproductive or developmental effects of capsaicin in people. Exposure to capsaicin causes increased airway resistance. People who suffer from asthma or other respiratory diseases may be more sensitive to capsaicin.

While children may be especially sensitive to pesticides compared to adults, there are currently no data to conclude that children have increased sensitivity specifically to capsaicin.

Most wildlife will avoid capsaicin because it has such a strong odor and taste. However, birds cannot taste capsaicin and will not be repelled by it. Capsaicin is toxic to bees and other beneficial insects. Researchers believe that capsaicin and similar compounds protect the seeds inside the peppers from fungus.