If adding Drug B to Drug A shifts the curve to the right, what effect does Drug B have on Drug A?

If adding Drug B to Drug A shifts the curve to the right, what effect does Drug B have on Drug A?

• A. Antagonistic
• B. Synergistic
• C. Additive
• D. Potentiating

Answer: (A)

Shifting the dose–response curve to the right indicates decreased potency of Drug A in the presence of Drug B. Potency is about the amount of Drug A needed to achieve a given effect; a rightward shift means you must use more of Drug A to reach that same level of effect. This pattern is classic for antagonism, especially competitive antagonism, where Drug B competes with Drug A at the same receptor. Because the antagonist blocks binding, higher doses of Drug A can overcome the blockade so the maximum effect is still achievable if you keep increasing Drug A. In contrast, synergistic or additive interactions would raise the overall effect even at the same doses, and potentiation would enhance Drug A’s effect beyond what would be expected. Those patterns don’t describe a rightward shift caused by an antagonist.

Shifting the dose–response curve to the right indicates decreased potency of Drug A in the presence of Drug B. Potency is about the amount of Drug A needed to achieve a given effect; a rightward shift means you must use more of Drug A to reach that same level of effect. This pattern is classic for antagonism, especially competitive antagonism, where Drug B competes with Drug A at the same receptor. Because the antagonist blocks binding, higher doses of Drug A can overcome the blockade so the maximum effect is still achievable if you keep increasing Drug A.

In contrast, synergistic or additive interactions would raise the overall effect even at the same doses, and potentiation would enhance Drug A’s effect beyond what would be expected. Those patterns don’t describe a rightward shift caused by an antagonist.