A neurotransmitter produced naturally in the human body which affects mood. 5-hydroxytryptamine (5-HT) or Serotonin is of no use taken orally as it cannot cross the blood brain barrier. However, supplements use precursors such as tryptophan ( amino acid ) and 5-hydroxytryptophan (5-HTP) ( metabolite ) to stimulate the metabolic pathway that produces serotonin in the body.