Capsaicin is a natural irritant to mammals which is why chillies produce it to protect themselves from being eaten. It causes a burning sensation to any tissue it comes into contact with especially the sensitive eyes and nasal passages which is why it is used in pepper sprays. However, when consumed internally, it has been proven to protect DNA and cells from attack by toxic molecules from tobacco and other toxins. It can also prevent cancer by inhibiting the transformation of cells which eventually form cancer.
Capsaicin can also help to fight off bacterial infections and used to relieve the pain of peripheral neuropathy and post-herpetic neuralgia caused by shingles (herpes zosta virus). Clinical trials have proven that it can successfully treat those suffering with nasal allergies such as allergic rhinitis when applied as a nasal spray. Capsaicin also increases metabolic activity promoting natural weight loss.
NOTE: Pepper sprays should be used sparingly because, in large quantities, capsaicin can cause death. Symptoms of overdose from inhalation include difficulty breathing, blue skin and convulsions. The large amount needed to kill an adult human and the low concentration of capsaicin in chillies make the risk of accidental poisoning by chilli consumption virtually impossible.