5-Hydroxytryptophan is a precursor of serotonin, which is a neurotransmitter regulating numerous important bodily functions, in particular brain and nerve function. • 5-Hydroxytryptophan is the immediate amino acid precursor to serotonin. • Approximately 70% of oral 5-HTP is converted to serotonin, compared with around 3% of L-tryptophan. • Serotonin may be converted in the body to melatonin, important in regulating the sleep-wake cycle. • 5-HTP is sourced from the plant Griffonia simplicifolia.