SSRI
Nerve cells talk to each other across tiny gaps by releasing chemical messengers, then quickly vacuuming them back up to reset for the next signal. An SSRI blocks the vacuum for one particular messenger, serotonin, so it lingers longer in the gap. Over weeks, this is thought to help lift mood in depression and ease anxiety.
The target is the serotonin transporter (SERT), a membrane protein that recaptures serotonin after release. SSRIs bind SERT and block this reuptake; the word 'selective' means they do this with little effect on the closely related transporters for noradrenaline and dopamine, unlike older, messier antidepressants.
Chemically these are diverse small molecules (fluoxetine, sertraline, citalopram, paroxetine) that share the functional ingredients of a SERT blocker rather than one rigid skeleton: typically an aromatic system and a basic amine positioned to engage the transporter's substrate site. Their selectivity is what made them far safer in overdose than the tricyclic antidepressants they largely replaced.
An honest caveat: the mood benefit builds slowly over weeks even though reuptake is blocked within hours, the full reason for this delay is still debated, and SSRIs carry their own side effects and a discontinuation syndrome if stopped abruptly.
Fluoxetine blocks SERT with high selectivity over the noradrenaline and dopamine transporters, which is precisely the property its name advertises.
Selectivity across a transporter family as the defining design feature.
The 'selective' in SSRI is a selectivity claim about the transporter family, not a promise of no side effects; it was a deliberate design improvement over the broad-acting tricyclics.