Substance P (SP) is a peptide that is highly conserved across the animal kingdom and is involved in a number of inflammatory and growth promoting processes. SP has a net positive charge at physiological pH, it is an amphiphilic peptide with positively charged residues at the N-terminus and hydrophobic residues at the C-terminus, this controls how it interacts with cell membranes. SP is relatively stable in plasma (several hours) but has a short half-life in tissues (seconds/minutes).

SP is encoded by the TAC1 gene and is a member of the tachykinin peptide hormone family. SP is expressed by many cell types including: neurons; astrocytes; microglia; epithelial cells; endothelial cells; immune cells such as T cells and macrophages; dendritic cells and eosinophils and some stem cells and progenitor cells. The huge variety of cell types expressing SP suggest it is involved in a wide variety of physiological and pathophysiological functions.

SP mediates its functions by interacting with members of the neurokinin (NK) family of G protein–coupled receptors with high selectivity. Among these, SP binds to NK1R with the highest affinity, this receptor is expressed in a wide range of tissue types.

[5-FAM]-Substance P contains 5-Carboxyfluorescein (5-FAM), a widely used green fluorescent tag.