Also known as 5-hydroxytryptamine. An example of a neurotransmitter, it is a naturally occurring substance closely related to naturally occurring and synthetic hallucinogenic tryptamines.

• Serotonin uptake inhibitor:
A drug that inhibits neuronal re-uptake of serotonin, and consequently prolongs its action. Drugs of this class have been reported to be capable of reducing alcohol consumption. Certain antidepressants inhibit both the uptake of serotonin and that of noradrenaline (norepinephrine).