If two drugs have the same EC50 but different Emax, which has greater efficacy?

If two drugs have the same EC50 but different Emax, which has greater efficacy?

• A. The one with lower EC50
• B. The one with higher EC50
• C. The one with lower Emax
• D. The one with higher Emax

Answer: (D)

When two drugs share the same EC50, they have similar potency—that is, the dose needed to reach half of their maximal effect is about the same. Efficacy, however, is about how high the effect can go. If one drug has a higher Emax, it can produce a greater maximal response than the other. Even though both reach half-max at the same concentration, the drug with the higher Emax can push the response to a higher ceiling, giving greater overall efficacy. The drug with a lower Emax can’t surpass its ceiling, no matter how much you increase the dose, so its efficacy is limited. Hence, the higher Emax corresponds to greater efficacy.

When two drugs share the same EC50, they have similar potency—that is, the dose needed to reach half of their maximal effect is about the same. Efficacy, however, is about how high the effect can go. If one drug has a higher Emax, it can produce a greater maximal response than the other. Even though both reach half-max at the same concentration, the drug with the higher Emax can push the response to a higher ceiling, giving greater overall efficacy. The drug with a lower Emax can’t surpass its ceiling, no matter how much you increase the dose, so its efficacy is limited. Hence, the higher Emax corresponds to greater efficacy.