Efficacy Estimand
Definition
The estimated treatment effect under conditions of full adherence — what the drug does when actually taken as prescribed. Contrasts with the treatment-regimen estimand.
Efficacy Estimand
The efficacy estimand is one of two main ways modern clinical trials describe treatment effect. It estimates the average treatment effect for participants who actually adhere to the assigned treatment regimen — answering the question, “What does this drug do when patients take it as prescribed?” The efficacy estimand typically excludes or models around data from participants who discontinued treatment or who initiated rescue medication.
The contrasting approach is the treatment-regimen estimand, which estimates the effect of the assigned treatment policy as a whole, including the consequences of discontinuation. Treatment-regimen estimates are typically smaller in magnitude because they incorporate the dilution from participants who stopped taking the drug.
Modern Phase 3 obesity trials, including retatrutide’s TRIUMPH-1 and TRIUMPH-4 and tirzepatide’s SURMOUNT-1, report both estimands side by side. In TRIUMPH-1, for example, retatrutide 12 mg produced -28.3% mean weight loss under the efficacy estimand and -25.0% under the treatment-regimen estimand. Both are statistically significant versus placebo. Cross-trial comparisons should always specify which estimand is being quoted, because mixing the two creates artificial differences between drugs.