A patient with obesity has an increased Vd for lipophilic drugs. How would this influence loading dose calculation?

• A. Loading dose decreases with greater Vd
• B. Loading dose remains the same regardless of Vd
• C. Loading dose increases with Vd
• D. Loading dose becomes unpredictable with high Vd

Answer: (C)

Loading dose is used to rapidly achieve the target plasma concentration and scales with the volume of distribution. The key relationship is D_L = C_target × Vd, meaning the amount of drug needed initially is proportional to how large the distribution space is. In obesity, lipophilic drugs distribute into adipose tissue, increasing Vd. As Vd grows, you must supply more drug upfront to reach and hold the same therapeutic plasma concentration. Clearance and elimination rate influence how long the drug stays in the body after the loading dose, not the initial amount needed to reach the target concentration. So, with a higher Vd in obesity for lipophilic drugs, the loading dose increases.