Which statement best describes the relationship between volume of distribution and loading dose when aiming for a target plasma concentration?

• A. Higher Vd requires lower loading dose.
• B. Loading dose is independent of Vd.
• C. Higher Vd requires higher loading dose to reach target concentration.
• D. Loading dose equals target concentration divided by bioavailability.

Answer: (C)

The main idea is that the loading dose is set to achieve a specific target plasma concentration, and how much you need depends on how widely the drug distributes in the body. Volume of distribution (Vd) links the amount of drug in the body to the plasma concentration. If the drug distributes widely (large Vd), the same total amount produces a lower plasma concentration, so you must give more drug to reach the target level. This relationship is captured by the formula LD = (Target concentration × Vd) / F, where F is bioavailability. For IV administration, F equals 1, so LD = Target concentration × Vd. Therefore, a higher Vd requires a higher loading dose to reach the desired plasma concentration.