Why might a drug have a higher apparent Vd in obese patients?

• A. Increased Adipose Tissue Provides More Reservoir for Lipophilic Drugs
• B. Decreased Tissue Binding
• C. Faster Clearance
• D. Decreased Protein Binding

Answer: (A)

The main concept is how body composition affects where a drug goes in the body. In obesity, the large amount of fat tissue provides a substantial reservoir for lipophilic drugs. Lipophilic (fat-loving) drugs readily partition into adipose tissue, so more of the drug moves out of the bloodstream and into fat stores. This means, for a given dose, the concentration in plasma drops more than in lean individuals, which makes the apparent volume of distribution larger. Because Vd links dose to plasma concentration, greater tissue storage translates into a higher Vd.

This also has practical implications: if clearance doesn’t change, the larger Vd lengthens the drug’s half-life, slowing elimination and prolonging effects. Hydrophilic drugs don’t partition into fat to the same extent, so their Vd doesn’t rise as much in obesity.