Serotonin

Serotonin — also called 5-hydroxytryptamine, or 5-HT — is a monoamine neurotransmitter and signalling molecule found throughout the body. In the brain it helps govern mood, sleep, appetite, and sensory perception. Outside the brain, the vast majority of the body's serotonin resides in the gut and in blood platelets, where it plays roles in digestion and vascular regulation.

It is synthesised from the dietary amino acid tryptophan in a two-step process, and broken down primarily by the enzyme monoamine oxidase (MAO). Because so many psychoactive substances act directly on serotonin pathways, it is one of the most studied molecules in pharmacology.

How it works · its role

Serotonin acts by binding to an unusually large family of receptors — at least fourteen subtypes, grouped into several classes — each associated with distinct physiological and psychological effects. The 5-HT₂A receptor is the primary driver of the perceptual and cognitive changes produced by classic psychedelics. The 5-HT₁A receptor is implicated in anxiety regulation and baseline mood. Other subtypes shape nausea, gut motility, and cardiovascular tone.

Once released into the synapse, serotonin is cleared by the SERT (SERT), which pumps it back into the releasing neuron and ends the signal. Psychoactive drugs alter this system in a few fundamental ways: blocking SERT so serotonin lingers in the synapse, triggering a rapid mass release, or binding directly to receptors in serotonin's place.

Relevance to substances & effects

A wide range of substance classes converge on serotonin signalling. SSRIs and related antidepressants block SERT, gradually raising synaptic serotonin levels over days to weeks. MDMA acts differently — it forces a rapid, large-scale serotonin release while also blocking reuptake, which underlies the acute emotional openness and warmth it produces.

Classic psychedelics — including LSD and psilocin (the active form of psilocybin) — are potent agonists at the 5-HT₂A receptor. Their binding there is thought to be the primary mechanism behind perceptual distortion, altered thought patterns, and the characteristic quality of a psychedelic experience. MAOIs, by blocking serotonin breakdown, raise baseline levels and profoundly amplify any additional serotonergic input.

Because so many substances act through overlapping points in this system, drug combinations that stack serotonergic mechanisms carry a meaningful interaction risk.

Clinical · risk note

When serotonin activity is pushed well above normal — most often by combining two or more serotonergic drugs — the result can be serotonin syndrome: a constellation of agitation, rapid heart rate, sweating, muscle rigidity or twitching, and, in severe cases, dangerously elevated body temperature.

The highest-risk combinations typically involve an MAOI alongside a serotonin releaser or reuptake inhibitor, though other pairings can trigger it too. Serotonin syndrome can escalate quickly and should be treated as a medical emergency.