How is the initial drug concentration after an IV bolus (C0) related to dose and Vd?

• A. C0 = Dose / Vd.
• B. C0 = Vd / Dose.
• C. C0 = Dose × Vd.
• D. C0 = Dose × k.

Answer: (A)

Key idea: after an IV bolus, the entire dose is instantly distributed into the body’s apparent volume of distribution. The concentration at time zero (C0) is then the dose divided by that volume: C0 = Dose / Vd. This makes sense in units because dose (mg) divided by Vd (L) gives concentration (mg/L). The volume of distribution is a proportionality factor linking amount in the body to plasma concentration, not a physical space you can measure directly. The other forms would not give a concentration with the correct units: multiplying dose by Vd gives mg·L (not concentration), multiplying by a rate constant would mix in a time-based unit, and dividing Vd by Dose would yield L/mg (not a concentration).