Another serious drawback of all antidepressants is the requirement for long-term administration prior to maximal therapeutic efficacy. Although some patients show a partial response within 1–2 weeks, in general one must reckon with a delay of 3–6 weeks before full efficacy is attained. In general, this delay to onset of action is attributed to a spectrum of long-term adaptive changes. These include receptor desensitization, alterations in intracellular transduction cascades and gene expression, the induction of neurogenesis, and modifications in synaptic architecture and signaling.