5-HTP (the common name for the compound 5-hydroxytryptophan) is a derivative of the amino acid tryptophan. A mood-enhancing chemical, 5-HTP has attracted a good deal of attention lately because of its ability to increase pain tolerance, induce sleep, and affect how hunger is perceived. Unlike many other supplements (and drugs) that have molecules too large to pass from the bloodstream into the brain, molecules of 5-HTP are small enough to do so. Once in the brain, they're converted into an important nervous system chemical, or neurotransmitter, called serotonin. In 1994, 5-HTP was introduced in the U.S.; 5-HTP is thought to pose significantly less risk of contamination than L-tryptophan. Europeans have been taking 5-HTP for decades to treat insomnia and depression.