AlphaMED Resources · https://alphamedicine.uk/Resources/glossary/efficacy-effectiveness

Efficacy vs effectiveness

Efficacy is how well a treatment works under ideal, controlled conditions. Effectiveness is how well it works in everyday care.

Updated 19 October 2026 (1 minute read)

Summary

  • Efficacy is the performance of an intervention under ideal and controlled circumstances, usually shown in a clinical trial.
  • Effectiveness is the performance of the same intervention under real world conditions, with the patients and settings of routine care.
  • A trial can show that a treatment works and still leave open how it does in a busy clinic, because the trial setting differs from routine care.
  • The same treatment can show high efficacy in trials and lower effectiveness in practice, so a study's result depends on which of the two it measured.

Interview questions

  • What is the difference between efficacy and effectiveness?
  • A new drug works well in a trial, but the results from local clinics are weaker. How would you explain that?
  • Is a trial result enough to show that a treatment should be used across the NHS, or is evidence from routine care needed as well?

Sources

  1. PubMed Centralpmc.ncbi.nlm.nih.gov/…/PMC3912314

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Related

Clinical trial A clinical trial is a research study that tests a treatment in people to find out whether it is safe and whether it works. Randomised controlled trial A randomised controlled trial is a study that puts people into groups by chance, so that a treatment can be compared with something else given to a control group. Validity vs reliability Validity is whether a test measures what it claims to measure. Reliability is whether it gives consistent results when repeated. Placebo A placebo is a fake or dummy treatment that looks the same as the real one and is given to the control group of a trial.

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