Evidence-appraisal glossary
Effectiveness versus efficacy
Efficacy is whether a treatment works under ideal, tightly controlled trial conditions; effectiveness is whether it works in everyday clinical practice with typical patients and imperfect adherence. A drug can prove efficacious in a strict trial yet deliver smaller real-world effectiveness once ordinary conditions apply.
Also called: efficacy vs effectiveness, explanatory versus pragmatic trials, real-world effectiveness.
What it is
Both terms describe whether an intervention benefits patients, but under different conditions. Efficacy asks: does it work under ideal conditions? These "explanatory" trials use narrow eligibility, closely monitored dosing, high adherence, and expert providers to isolate the biological effect. Effectiveness asks: does it work in usual care? These "pragmatic" trials enroll broader, sicker, more varied patients and let treatment unfold as it would in routine practice. Most trials sit somewhere along this explanatory-to-pragmatic continuum.
How to use it when reading a study
Check which question the study actually answered before applying results to ordinary patients. Look at eligibility criteria, the care setting, how adherence was supported, and the comparator. Tightly selected samples and intensive monitoring signal efficacy, so the real-world effectiveness may be smaller. Tools like PRECIS-2 grade how pragmatic a design is. Effect sizes from ideal conditions are best read as an upper bound, not a promise of everyday performance.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.