By adding Drug B to Drug A, the curve shifts to the left. This indicates that Drug B had what effect on Drug A?

By adding Drug B to Drug A, the curve shifts to the left. This indicates that Drug B had what effect on Drug A?

• A. Antagonism
• B. No interaction
• C. Decrease potency
• D. Potentiation or an additive effect

Answer: (D)

A leftward shift in a dose–response curve means Drug B makes Drug A more potent, because the same effect is achieved with a smaller dose of Drug A. Potency is about the amount of Drug A needed for a given response; when Drug B is present, you require less of Drug A to reach that effect, indicating increased potency. This can arise from potentiation, where Drug B enhances Drug A’s effect, or from an additive interaction that still results in a greater effect at lower Drug A doses. Antagonism would reduce Drug A’s effect (often shifting the curve to the right or lowering the maximum effect), no interaction would leave the curve unchanged, and a decrease in potency would shift the curve to the right, not left.

A leftward shift in a dose–response curve means Drug B makes Drug A more potent, because the same effect is achieved with a smaller dose of Drug A. Potency is about the amount of Drug A needed for a given response; when Drug B is present, you require less of Drug A to reach that effect, indicating increased potency. This can arise from potentiation, where Drug B enhances Drug A’s effect, or from an additive interaction that still results in a greater effect at lower Drug A doses. Antagonism would reduce Drug A’s effect (often shifting the curve to the right or lowering the maximum effect), no interaction would leave the curve unchanged, and a decrease in potency would shift the curve to the right, not left.