Of a drug or treatment: The maximum ability of a drug or treatment to produce a result regardless of dosage. A drug passes efficacy trials if it is effective at the dose tested and against the illness for which it is prescribed. In the procedure mandated by the FDA, Phase II clinical trials gauge efficacy and Phase III trials confirms it. See Phase II and III Trials .

In medicine:  Is the ability of an intervention or drug to reproduce a desired effect in expert hands and under ideal circumstances.