How long does it take for antidepressant therapies to act?

OBJECTIVE: To review the proposition that antidepressants have a delayed onset of action by employing measurement and analytic strategies that overcome problems confounding interpretation of many efficacy studies.
METHOD: A subset of patients was recruited to the longitudinal component of the Australasian database study, was assessed at baseline, and then completed measures of depression and anxiety when treatment commenced, and every 3 days over the next 4 weeks. The trajectories of defined 4-week outcome responders and non-responders were compared.
RESULTS: Both groups showed a similar decrease in depression (and anxiety) over the first 3 days. A clear trend break then occurred, with little further improvement in the non-responders, as against distinct and progressive improvement in the responders. Ongoing early improvement (across days 3-6) was a strong predictor of responder status.
CONCLUSIONS: The small sample size limits firm interpretation, although distinct interpretive advantages to the study design are evident. Findings are compatible with a number of recent studies arguing against any extensive delayed onset of action for the antidepressant drugs, but argue for caution in interpreting immediate improvement as predicting likely responder status, and more for examining early and sustained improvement as such a marker.