What do antidepressants actually do?
They alter neurotransmitter signalling, most commonly serotonin — and the honest position is that how that produces improvement in mood is not fully understood, which is worth stating plainly because the simple explanation people were given is not well supported.
The "chemical imbalance" account. Depression was widely explained as low serotonin, corrected by raising it. This was always a simplification and is not supported as a general explanation — a prominent review found no consistent evidence that depression is caused by low serotonin. That does not mean antidepressants do not work; it means the mechanism was mis-described.
The timing problem that reveals it. SSRIs raise synaptic serotonin within hours, while mood improvement takes weeks. If raising serotonin were the mechanism, the effect would be immediate. Current thinking focuses on downstream changes — receptor adaptation, neuroplasticity, BDNF expression, and altered emotional processing appearing before mood changes.
The main classes: SSRIs, generally first-line; SNRIs; tricyclics, effective and with more side effects; MAOIs, now rarely used; and several others with distinct mechanisms used for particular presentations.
What the evidence shows about effectiveness:
They work better than placebo, consistently, in meta-analyses of many trials.
The average effect is modest, and larger in more severe depression — the difference from placebo is smallest in mild depression, which is why guidelines suggest therapy first there.
Placebo response in depression trials is high, which is a genuine finding rather than an argument that the drugs are inert.
Response varies enormously, and finding the right drug frequently takes more than one attempt.
What people should know practically:
They take weeks to work, and side effects frequently appear first — which is why many people stop before any benefit.
Do not stop abruptly. Discontinuation symptoms are real and can be severe, and tapering should be slow and supervised.
They are not addictive in the sense of craving or dose escalation, and they can cause discontinuation effects, which are different things.
Combining with therapy outperforms either alone in moderate to severe depression.
General information, not medical advice.