A 60 kg patient receives a 120 mg IV bolus; Vd = 40 L. What is C0?

• A. 0.3 mg/L
• B. 12 mg/L
• C. 3.0 mg/L
• D. 4.0 mg/L

Answer: (C)

For an IV bolus in a single-compartment view, the initial concentration (C0) equals the dose divided by the volume of distribution: C0 = Dose / Vd. The bolus puts the entire dose into the distribution volume instantly, so concentration at time zero is simply how much drug per liter of distribution space.

Here, Dose = 120 mg and Vd = 40 L, so C0 = 120 mg / 40 L = 3 mg/L. The patient’s weight isn’t needed for this calculation.

This result corresponds to the given Vd. The other numbers would require different Vd values (for example, 0.3 mg/L would need a Vd of 400 L, 12 mg/L would need 10 L, and 4.0 mg/L would need 30 L).