Loading dose example: If target Css is 4 mg/L and Vd is 40 L, what loading dose should be given?

• A. 160 mg
• B. 40 mg
• C. 4 mg
• D. 400 mg

Answer: (A)

A loading dose is used to rapidly reach the target plasma concentration by delivering enough drug to fill the volume of distribution to that level. The amount needed is D_L = Css × Vd. With a target Css of 4 mg/L and a Vd of 40 L, the loading dose is 4 mg/L × 40 L = 160 mg. This brings the initial concentration up to about 4 mg/L right away. The other amounts don’t match the needed Css when distributed into 40 L. If the drug weren’t given IV or isn’t fully bioavailable, a bioavailability factor would adjust this (D_L = Css × Vd / F), but under the usual IV bolus assumption, 160 mg is correct.