If the curve shifts to the right and the maximum effect is reduced, what is the drug interaction?

• A. Synergistic
• B. Antagonistic with reduced maximum effectiveness
• C. Antagonistic
• D. No interaction

Answer: (B)

When a drug interaction changes both potency and efficacy, you’re looking at how the presence of one drug affects the other’s ability to produce a response. A rightward shift on a dose–response curve means you need a higher dose to achieve the same effect, so potency is reduced. If the maximum achievable effect is also lowered, that means the interaction is diminishing the overall effectiveness of the drug, i.e., reducing efficacy. Together, these changes point to an antagonistic interaction that lowers maximal effectiveness—classic noncompetitive (insurmountable) antagonism, where the antagonist blocks receptor function in a way that cannot be fully overcome by increasing the agonist. The other scenarios don’t fit: synergistic would shift the curve left and raise the max, while no interaction would show no change, and competitive antagonism typically shifts the curve to the right without reducing the maximum.