Effectiveness refers to how well a therapy provides the expected therapeutic effect on a disease or symptom in clinical practice in the real world. Phase 4 clinical trials and other post-market surveillance studies are usually designed as effectiveness trials. Although sometimes used interchangeable, efficacy has a different meaning than effectiveness in the therapy regulatory and marketing approval process.
A Primer on Effectiveness and Efficacy Trials [2014 published article]
Criteria for Distinguishing Effectiveness From Efficacy Trials in Systematic Reviews [Technical Reviews, No 12. Agency for Healthcare Research and Quality (US), 2006 Apr]