5-Methoxytryptophan (5-MTP), or 5-methoxy-<small>L</small>-tryptophan, also known as 5-methoxy-ñ-carboxytryptamine or as cytoguardin, is an endogenous metabolite of tryptophan with anti-inflammatory and anti-fibrotic effects. It is biosynthesized in two steps by tryptophan hydroxylase (TPH) and then by hydroxyindole O-methyltransferase (HIOMT), with 5-hydroxytryptophan (5-HTP) as a metabolic intermediate. 5-MTP is also a metabolic precursor of 5-methoxytryptamine. The compound promotes sleep in rodents similarly to melatonin. In contrast to melatonin, 5-MTP did not show affinity for melatonin receptors. It was first described in the scientific literature by at least 1951.