Define and distinguish loading dose and maintenance dose, including formulas with Css and CL/Vd.

• A. Loading dose = Css × Vd / F; Maintenance dose per interval = Css × CL × τ / F
• B. Loading dose = Css × Vd × F; Maintenance dose per interval = Css × CL × τ / F
• C. Loading dose = Css / (Vd × F); Maintenance dose per interval = Css × Vd × τ / CL
• D. Loading dose = Css × CL / F; Maintenance dose per interval = Css × Vd × τ / F

Answer: (A)

The key idea is that you use two different dosing concepts to reach and then hold a target plasma level (Css). To reach Css quickly, you give a loading dose that essentially fills the body’s distribution space. Since the amount of drug in the body at steady state is Css × Vd, the loading dose must supply that amount, and you divide by bioavailability F to account for what actually reaches systemic circulation when given by the chosen route: Loading dose = Css × Vd / F.

To keep Css steady with ongoing dosing, you replace the drug that’s eliminated during each dosing interval τ. The amount eliminated during that interval is Css × CL × τ, and again you divide by F to reflect oral bioavailability: Maintenance dose per interval = Css × CL × τ / F. This ensures the input rate matches the elimination rate, maintaining Css over time.

So the best formulas are loading dose = Css × Vd / F and maintenance dose per interval = Css × CL × τ / F. For IV dosing (F = 1), the loading dose simplifies to Css × Vd, and the maintenance dose per interval becomes Css × CL × τ.