What is 5-htp? 5-HTP is a precursor to serotonin, the neurotransmitter sometimes touted as responsible for well-being. 5-HTP is an easy way to increase brain serotonin levels by bypassing rate-limiting steps, and users can receive the reward or harm of increased brain serotonin. 5-HTP is a compound that is converted into serotonin in the brain. Serotonin is one of the …