Potency refers to the amount or dose of a drug required to produce a given effect, according to Merck. Common measurements of dosage include milligrams and milliliters. Potency is a way of comparing the effectiveness of different drugs.
Efficacy is used to describe how well a drug can produce a therapeutic response. A drug for hypertension that lowers blood pressure more than another has greater efficacy, for example.
Other factors besides the potency and efficacy of a drug help determine the selection of a drug, according to Merck. Those factors include cost, side effects, dose frequency, toxicity concerns and duration of effect.
The term drug potency refers to the strength of a drug. Efficacy refers to its effectiveness in treatment. Both terms help to evaluate the effects of a given drug, according to Merck.com.