Which statement best differentiates potency from efficacy?

Which statement best differentiates potency from efficacy?

• A. Potency is the amount of drug needed to produce a given effect; efficacy is the maximal effect achievable.
• B. Potency is duration of action; efficacy is side-effect profile.
• C. Potency is the receptor affinity; efficacy is the solubility.
• D. Potency equals the maximal effect; efficacy equals the dose for half-max response.

Answer: (A)

Potency and efficacy describe different aspects of how a drug produces a response. Potency is about the amount of drug needed to produce a given effect—the lower the dose required, the more potent the drug. Efficacy is about the maximum effect the drug can achieve, the ceiling of its response, regardless of how much you dose. So the best statement is the one that says potency is the amount of drug needed to produce a given effect, and efficacy is the maximal effect achievable. It captures the distinct meanings: potency cares about dose proportion, while efficacy cares about the size of the response possible. The other ideas mix in things like duration of action, side-effect profiles, receptor affinity, or solubility, or they swap the definitions (maximal effect for potency, dose for half-max response for efficacy), which aren’t what potency and efficacy mean.

Potency and efficacy describe different aspects of how a drug produces a response. Potency is about the amount of drug needed to produce a given effect—the lower the dose required, the more potent the drug. Efficacy is about the maximum effect the drug can achieve, the ceiling of its response, regardless of how much you dose.

So the best statement is the one that says potency is the amount of drug needed to produce a given effect, and efficacy is the maximal effect achievable. It captures the distinct meanings: potency cares about dose proportion, while efficacy cares about the size of the response possible.

The other ideas mix in things like duration of action, side-effect profiles, receptor affinity, or solubility, or they swap the definitions (maximal effect for potency, dose for half-max response for efficacy), which aren’t what potency and efficacy mean.