The magnitude of a drug's response is proportional to its concentration at which site?

• A. Site of Administration
• B. Site of Action
• C. Blood
• D. Lymph

Answer: (B)

The magnitude of a drug’s response is determined by how much drug is present at the site where it works, the site of action. The drug must reach its target—usually a receptor or other molecular site—and bind to it; the more occupancy of those targets, the greater the response, up to a limit. This relationship is described by how the effect grows with concentration at the site of action (often modeled by receptor occupancy and dose–response curves, with higher local concentration leading to more target engagement). Blood or plasma levels are useful for tracking overall exposure, but they don’t always mirror the actual concentration at the target tissue due to distribution, binding, and barriers that influence tissue penetration. Therefore, the concentration at the site of action best explains why the response scales with dose.