Substance P is a bioactive 11-amino acid peptide first isolated in 1931 from brain and intestine. The peptide is involved in many physiological processes including pain modulation, smooth muscle contraction, blood pressure control, kidney function, and water homeostasis. Substance P is widely distributed in numerous tissues and body fluids including the central and peripheral nervous system, gastrointestinal tract, respiratory tract, visual system, and circulatory system.