5-HTP (5-Hydroxytryptophan) is a drug-free and naturally occurring plant-derived amino acid that participates in the body’s synthesis of serotonin. 5-HTP is the direct precursor to serotonin. 5-HTP works in the brain and central nervous system by increasing the production of the chemical serotonin. Serotonin can affect sleep, appetite, diet, temperature, and sexual behavior. Serotonin is the chemical messenger that plays a key role in controlling appetite, insomnia, and enhancing mood.