What is the difference between efficacy and potency?

• A. Efficacy = Dose; Potency = Duration
• B. Efficacy = Half-Life; Potency = Distribution
• C. Efficacy = How Well a Drug Does Its Job; Potency = How Much of a Drug Is Needed to Get the Job Done
• D. Efficacy = Side Effects; Potency = Absorption

Answer: (C)

The main idea here is telling the difference between how strong a drug’s effect can be and how much of the drug is needed to achieve that effect.

Efficacy is about the maximum effect a drug can produce. If a drug has high efficacy, it can achieve a strong or near-full therapeutic effect when given in adequate amounts. It speaks to the magnitude of the effect, not the amount you have to give.

Potency, on the other hand, is about the amount of drug required to reach a given level of effect. A more potent drug needs a smaller dose to achieve the same effect as a less potent one. Potency relates to how far the dose-response curve sits to the left or right, with leftward curves indicating higher potency.

Two important points: efficacy and potency are independent. You can have a drug with high efficacy but low potency (it can produce a strong effect but requires a larger dose), or high potency with lower efficacy (it needs only a small dose but can’t produce a very large effect). This distinction is exactly what the correct choice captures: efficacy = how well the drug does its job (maximum effect), potency = how much of the drug is needed to get the job done.