What is the definition of potency in pharmacology?

• A. The concentration required to achieve 50% of the maximal effect.
• B. The maximal effect a drug can produce.
• C. The speed of onset of action.
• D. The selectivity for a receptor subtype.

Answer: (B)

Potency refers to the amount of drug needed to produce a given level of response. On a dose–response curve, it’s about where the curve sits along the dose axis: a more potent drug reaches the same level of effect at a lower dose, so its curve is shifted to the left. The common way we quantify this is by EC50, the concentration that yields 50% of the maximal effect. This concept helps compare drugs that may achieve the same maximum effect (efficacy) but require different amounts to get there.

The maximal effect a drug can produce is called its efficacy, which is about the height of the curve, not the dose needed to reach it. The other options refer to different ideas: onset of action is about how quickly the drug starts working, and selectivity describes preference for a receptor subtype.