At first glance, it would seem the only difference between “effectiveness” and “efficacy” is a handful of letters. But these terms actually have very different meanings in the context of medical research and should never be used interchangeably. Efficacy refers to how well a drug, device or other intervention performs under ideal conditions. Efficacy can only be determined in clinical trials, which typically have very specific criteria for participants (and therefore may not be generalizable to the general public).
Deeper dive
The criteria for clinical trial participants nearly always have age requirements and often have sex, gender, race or ethnicity requirements as well. Other potential criteria could relate to having or not having certain underlying conditions, having a certain severity of a particular disease, living in a particular geographical area, living in a rural or urban or suburban area or having public, private or no insurance, among other things. The stricter the criteria is, the less generalizable the findings might be. If, for example, a trial includes only Black women in their 40s who were diagnosed with type 2 diabetes within the past two years, would the effects of a drug being tested also apply to white men in their 20s who have had type 2 diabetes since they were children? It’s often impossible to know without doing a trial with that population as well.
Effectiveness, on the other hand, is how well a drug, device or intervention performs in everyday real life for a broader range of patients. Data from effectiveness studies looks at the intervention’s effects in a large, often diverse population. There may still be criteria for inclusion in the study, but it generally will be broader and it often may include people who are sicker or healthier than the groups included in efficacy trials. Effectiveness has much more relevance for doctors and other clinicians.
Why not just always test the effectiveness of an intervention then? The problem with this approach is that the diversity of characteristics among participants could mask potential therapeutic effects or side effects because it’s harder to tell whether the outcomes occur because of the drug or because of differences among the participants.
Another difference between efficacy and effectiveness relates to how a drug is prescribed and how well the patients adhere to it, or how different physicians might perform different procedures. An efficacious drug may not actually be effective the way it’s used. Further, efficacy trials are designed to answer the question of whether a drug, device or intervention works at all while effectiveness studies help answer whether that intervention can help patients. In this sense, efficacy relies more on results’ statistical significance whereas effectiveness relates more to results’ clinical significance. Read more on efficacy and effectiveness here and here.