Substance P is involved in the transmission of pain impulses from peripheral receptors to the central nervous system. It has been theorized that it plays a part in fibromyalgia. Capsaicin has been shown to reduce the levels of Substance P probably by reducing the number of C-fibre nerves or causing these nerves to be more tolerant. Capsaicin is a Substance P antagonist. It acts to reduce the level of Substance P. In the central nervous system, Substance P has been associated with the regulation of mood disorders, anxiety, stress, reinforcement, neurogenesis, respiratory rhythm, neurotoxicity, nausea, emesis (vomiting), and pain.Tachykinins tend to speed up the neural firing rate, to improve the response time in the face of danger. The psychological effect is that things appear to happen in slow motion. You feel you have more time — more clock cycles — to figure out what to do. Capsaicin is the hot stuff in chili peppers and Mongolian Fire Oil.
One of the more interesting beneficial effects of Substance P is neurogenesis. Evidently, Substance P causes the brain to generate brand new neurons, perhaps to enable the brain to host new adaptations to cope with persistent stressful and problematic situations.