Binding of drugs to plasma proteins has the greatest impact on:

• A. Absorption
• B. Distribution
• C. Metabolism
• D. Excretion

Answer: (B)

Binding to plasma proteins mainly determines how widely a drug spreads through the body. When a drug attaches to proteins like albumin, that portion cannot cross capillary membranes, so only the unbound fraction can leave the bloodstream to reach tissues. This means the degree of binding directly limits distribution and sets the apparent volume of distribution, since most of the drug remains in plasma as bound reservoir while a smaller free portion populates tissues. Absorption occurs before binding, and metabolism and excretion rely on the free drug that is available for enzymes and for clearance, respectively. So the greatest impact of protein binding is on distribution.