Why do antidepressants take weeks to work?

SSRIs increase synaptic serotonin within hours, but clinical benefit typically takes 4-8 weeks. The discrepancy suggests the primary mechanism involves downstream changes — neuroplasticity, neurogenesis, or network-level reorganisation.

Why ketamine (a non-SSRI) produces antidepressant effects within hours sharpens the puzzle. A clear mechanistic account remains open.