A loading dose is administered to achieve immediate therapeutic plasma concentration. The volume of distribution (Vd) is the primary pharmacokinetic parameter used to calculate the loading dose. This is because Vd reflects:

• A The rate at which the drug is eliminated by the liver and kidney
• B The fraction of oral dose that reaches systemic circulation after first-pass metabolism
• C The time taken for the drug to reach steady-state plasma concentration
• D The total amount of drug required to produce the target plasma concentration, regardless of where the drug is distributed ✓

Explanation

Loading dose = target plasma concentration × Vd / bioavailability. The Vd relates the amount of drug in the body to the plasma concentration (Cp = Amount / Vd), and therefore the total drug dose needed to achieve any given target Cp is directly proportional to Vd. The elimination rate constant and clearance determine maintenance dose and half-life; bioavailability is incorporated when oral loading doses are calculated.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.