What determines the site of a drug's action?

• A. Location and functional capacity of its receptors; does not necessarily depend on drug distribution
• B. The drug's color and taste
• C. The route of administration
• D. The time of day it is taken

Answer: (A)

The site of a drug’s action is determined by where its target receptors or enzymes are located and capable of producing a response, and by the drug’s ability to bind with enough affinity at those sites. Even though a drug may circulate widely, it only exerts effects where the target is present and functional, so receptor location and capacity define where the action occurs. Distribution influences how much drug reaches those locations (exposure) but does not by itself set the action site.

Color and taste have no impact on where the drug acts; they don’t determine pharmacologic targets. Route of administration affects how the drug is absorbed and distributed, influencing onset and intensity, but it does not decide the intrinsic site of action. Time of day can affect pharmacokinetics or receptor sensitivity in some contexts, but it doesn’t establish the site where the drug acts.