Serotonin (5-hydroxytryptamine, 5-HT) is a chemical found in the human body.

It carries signals along and between nerves - a neurotransmitter. It is mainly found in the brain, bowels and blood platelets.

Serotonin is thought to be especially active in constricting smooth muscles, transmitting impulses between nerve cells, regulating cyclic body processes and contributing to wellbeing and happiness.

Serotonin is regarded by some researchers as a chemical that is responsible for maintaining mood balance, and that a deficit of serotonin leads to depression.