How would loading dose be calculated given a target Cp and Vd?

How would loading dose be calculated given a target Cp and Vd?

• A. Load dose equals Cp × Vd divided by bioavailability.
• B. Load dose equals Vd × Cp; adjust for bioavailability if not IV; for IV it's Cp target × Vd.
• C. Load dose equals Vd plus Cp.
• D. Load dose equals Cp divided by Vd.

Answer: (B)

The key idea is to deliver a dose that “fills” the body’s apparent distribution space to reach the target plasma concentration quickly. After a bolus that distributes instantly, the concentration in plasma is approximately Dose divided by the volume of distribution (Cp ≈ Dose / Vd). Rearranging gives Dose ≈ Cp × Vd. When the route is intravenous, the bioavailability F is 1, so the loading dose simplifies to Cp × Vd. If the drug is not given IV, you must account for bioavailability and use Dose = Cp × Vd / F. That’s why the best answer expresses the loading dose as Vd × Cp and notes to adjust for bioavailability if not IV. The other forms either mix up the relationship (e.g., adding Vd to Cp) or use incorrect algebra (e.g., Cp divided by Vd), which would not yield a correct dose or units.

The key idea is to deliver a dose that “fills” the body’s apparent distribution space to reach the target plasma concentration quickly. After a bolus that distributes instantly, the concentration in plasma is approximately Dose divided by the volume of distribution (Cp ≈ Dose / Vd). Rearranging gives Dose ≈ Cp × Vd. When the route is intravenous, the bioavailability F is 1, so the loading dose simplifies to Cp × Vd. If the drug is not given IV, you must account for bioavailability and use Dose = Cp × Vd / F.

That’s why the best answer expresses the loading dose as Vd × Cp and notes to adjust for bioavailability if not IV. The other forms either mix up the relationship (e.g., adding Vd to Cp) or use incorrect algebra (e.g., Cp divided by Vd), which would not yield a correct dose or units.