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Efficacy refers to how well a therapy provides the expected therapeutic effect on a disease or symptom under ideal and controlled circumstances. Phase 1 to 3 clinical trials are efficacy trials in the vast majority of cases. Although sometimes used interchangeable, effectiveness has a different meaning than efficacy in the therapy regulatory and marketing approval process.

Sourced From
A Primer on Effectiveness and Efficacy Trials [2014 published article]
Phase IV of Drug Development [2010 published article]
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Criteria for Distinguishing Effectiveness From Efficacy Trials in Systematic Reviews [Technical Reviews, No 12. Agency for Healthcare Research and Quality (US), 2006 Apr]

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