Which expression correctly defines maintenance dose to maintain Css?

• A. MD = Css × Cl × τ.
• B. MD = Css × Cl × τ × F.
• C. MD = Css × Vd × τ.
• D. MD = Css × Cl / τ.

Answer: (A)

Maintaining a target steady-state concentration depends on replacing what is removed in each dosing interval. The amount eliminated over a dosing interval τ at steady state is Cl × Css × τ. To keep Css, the dose given each interval must equal that amount.

For intravenous administration, bioavailability F is 1, so the maintenance dose per interval is MD = Css × Cl × τ.

If the drug is given by another route, you must account for incomplete absorption by dividing by F: MD = Css × Cl × τ / F. The other expressions don’t fit because using Vd in this context isn’t what sets the daily or interval dose, and placing τ in the denominator or multiplying by F would misestimate the amount needed.

So the expression MD = Css × Cl × τ is correct for IV administration.