True or false: Loading dose equals Css,inf times Vd.

• A. False; Loading dose equals Css,inf times Vd
• B. True; Loading dose equals Css,inf times Vd
• C. False; Loading dose equals Css,inf divided by Vd
• D. False; Loading dose equals Css,inf divided by Cl

Answer: (B)

Loading dose is used to quickly reach the target steady-state concentration by delivering an amount that, when distributed in the body, yields that concentration. For an IV bolus, right after the dose the concentration is D/Vd. To hit the desired Css,inf, set D/Vd = Css,inf, which gives D = Css,inf × Vd. That’s why the loading dose equals Css,inf times Vd. In linear pharmacokinetics, a maintenance rate would then be Css,inf × Cl to keep the concentration at Css,inf. If distribution isn’t instantaneous or kinetics are nonlinear, adjustments are needed, but under standard assumptions this relationship holds.