TBW-based dosing is not universally correct; it overcorrects for which type of drugs?

• A. Hydrophilic drugs
• B. Lipophilic drugs
• C. All drugs
• D. No drugs

Answer: (A)

The main idea is how body composition affects drug distribution. Total body water–based dosing assumes a higher weight means a proportionally larger distribution volume. In obesity, much of the extra weight is fat, which contains little water, so hydrophilic drugs (which stay in water-rich compartments) don’t actually have as large a distribution volume as TBW-based calculations would imply. Dosing by TBW therefore tends to deliver a higher dose than needed for hydrophilic drugs, raising the risk of toxicity. Lipophilic drugs, which distribute into fat, don’t fit TBW-based dosing as well, but the issue here is about overestimating doses for water-soluble drugs, not those that partition into fat.