In medical terms, efficacy refers to the ability of a product to provide a beneficial effect.
In the context of healthcare, the efficacy of a product to yield therapeutic benefits is explored. Interventions that may be explored include drugs, medical devices, surgical procedures and public health interventions
The intervention is compared to other interventions available at the time and efficacy is established if the new intervention is at least as good as the others available. Typically, these types of comparisons are made in randomized clinical trials.
In pharmacology, efficacy describes the maximum response that can be achieved with a drug. The effect of the drug is plotted against dose in a graph, to give the dose–response curve. The increasing doses used are displayed by the X axis and the half maximal and maximal responses are displayed by the Y axis. The highest point on the curve shows the maximum response (efficacy) and is referred to as the Emax.
The effectiveness of a drug is considered in two distinct ways, “method effectiveness” and “use” effectiveness. Method effectiveness refers to the maximum response that is achieved when the drug is taken exactly as prescribed, while use effectiveness describes the response obtained when the drug is used under typical circumstances, when adherence to the drug may not be 100%.
The use effectiveness which is typically assessed during “intention to treat” analysis of clinical studies is often biased compared to the findings obtained when method effectiveness is used.