... its ability to increase pain tolerance, induce sleep, and affect how hunger is perceived. Unlike many other supplements (and drugs) that ... 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 ...