Learning Objective
By the end of this lesson, learners will be able to interpret parallel and nonparallel dose–response curves, distinguish when two drugs act on the same versus different receptors, and accurately compare affinity, potency, and efficacy between drugs based on curve shape, position, and maximal response. Learners will also develop the ability to apply these concepts to exam-style questions, including USMLE Step 1 scenarios.
Teaching Note
- Parallel slopes indicate that two drugs act on the same receptor/mechanism.
- Nonparallel slopes indicate that drugs act on different receptors.
- Affinity can only be compared if drugs act on the same receptor.
- Potency compares the dose required to achieve an effect.
- Efficacy compares the maximal effect achieved.
Using the log dose-response curves:
→ Drugs A and B are parallel → same mechanism.
→ Drugs X and Y differ → different mechanisms.
→ A has a higher affinity than B because its curve is closer to the y-axis.
→ A more potent than B; X more potent than Y.
→ A and B have equal efficacy; X has greater efficacy than Y.
