The magnitude of therapeutic response is proportional to concentration of drug at which site?

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

Answer: (B)

The therapeutic effect is determined by how much drug actually reaches and interacts with its target at the site of action. When the drug concentration at that site increases, more target sites are engaged, producing a greater effect until saturation or other limits are reached. Blood concentration reflects overall exposure but not the exact amount at the action site, because distribution and barriers determine how much drug gets to the target. The site of administration affects how quickly the drug enters the body, not the final magnitude of the response. Urine relates to elimination, not the immediate pharmacodynamic effect. So the key determinant of how strong the response will be is the concentration at the site of action.