Which expression defines a loading dose in relation to target Css and volume of distribution?

• A. Loading dose is LD = Css × Cl.
• B. Loading dose is LD = Css × Vd × F.
• C. Loading dose is LD = F × Css × Vd.
• D. Loading dose is LD = Css × Vd.

Answer: (D)

Loading dose is the amount of drug needed to promptly reach a target steady-state concentration by filling the body’s apparent volume of distribution. The key idea is that how much drug you must give initially depends on how widely the drug distributes in the body (Vd). If the drug is given intravenously with complete bioavailability (F = 1), the amount required to achieve the target Css is simply Css multiplied by Vd. In other routes where bioavailability is not 100%, you divide by F, giving LD = Css × Vd / F. The formulation that multiplies by F would overestimate the needed load when F is less than 1, and using clearance (Cl) relates to elimination, not the initial amount to achieve Css. Hence, Css × Vd is the appropriate expression under IV administration.