Serotonin is manufactured in the brain and the intestines. The majority of the body's serotonin, between 80-90%, can be found in the gastrointestinal (GI) tract. It can also be found in the blood platelets and the central nervous system (CNS).

As serotonin can be found widely across the body, it is believed that the chemical plays a role in influencing a variety of body and psychological functions.

Serotonin cannot cross the blood-brain barrier, therefore, serotonin that is used inside the brain must be produced within it.