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.
However, capsaicin is also the most potent way to inhibit the vagus nerve which can have some adverse health implications. The vagus nerve, or the pneumogastric nerve, allows the brain to monitor and receive information about several of the body's different functions. It is the tenth cranial nerve which connects the brain stem to the body and supplies parasympathetic information to control the heart, lungs and digestive tract. It is also involved in many vital hormone secretions and is therefore important that it does not get compromised too often.
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.
However, the large amount needed to kill an adult human and the amount of chillies needed, to consume enough capsaicin, make the risk of accidental poisoning by eating chilli peppers virtually impossible.