Which formula correctly describes a loading dose for a non-intravenous administration given target Cp, Vd, and bioavailability F?

Which formula correctly describes a loading dose for a non-intravenous administration given target Cp, Vd, and bioavailability F?

• A. Load dose equals Cp × Vd × F.
• B. Load dose equals Vd × Cp.
• C. Load dose equals (Cp × Vd) / F.
• D. Load dose equals Cp divided by Vd.

Answer: (C)

Achieving the target plasma concentration quickly requires a loading dose that accounts for how much drug actually gets into the bloodstream. For a non-intravenous route, only a fraction F of the administered dose reaches systemic circulation. The amount that must be present in the body at steady state is Cp multiplied by Vd. So the administered dose must satisfy Dose × F = Cp × Vd. Solving for Dose gives Dose = Cp × Vd / F. This is why dividing by F is essential for non-IV routes: it compensates for incomplete absorption. For example, if the target Cp is 10 mg/L and the apparent volume of distribution is 50 L, with bioavailability F of 0.6, the loading dose would be 10 × 50 / 0.6 ≈ 834 mg. If the drug were given IV (F ≈ 1), the formula simplifies to Dose = Cp × Vd, which aligns with IV loading scenarios. This concept emphasizes that a larger Vd requires a larger dose to reach Cp, and lower bioavailability (smaller F) requires further adjustment by dividing by F.

Achieving the target plasma concentration quickly requires a loading dose that accounts for how much drug actually gets into the bloodstream. For a non-intravenous route, only a fraction F of the administered dose reaches systemic circulation. The amount that must be present in the body at steady state is Cp multiplied by Vd. So the administered dose must satisfy Dose × F = Cp × Vd. Solving for Dose gives Dose = Cp × Vd / F. This is why dividing by F is essential for non-IV routes: it compensates for incomplete absorption.

For example, if the target Cp is 10 mg/L and the apparent volume of distribution is 50 L, with bioavailability F of 0.6, the loading dose would be 10 × 50 / 0.6 ≈ 834 mg. If the drug were given IV (F ≈ 1), the formula simplifies to Dose = Cp × Vd, which aligns with IV loading scenarios.

This concept emphasizes that a larger Vd requires a larger dose to reach Cp, and lower bioavailability (smaller F) requires further adjustment by dividing by F.