The global epidemic of obesity has led to an increased prevalence of chronic diseases and need for pharmacological intervention. However, little is known about the influence of obesity on the drug exposure profile, resulting in few clear dosing guidelines for the obese. Here we present a semi-mechanistic model for lean body weight (LBW) that we believe is sufficiently robust to quantify the influence of body composition on drug clearance, and is therefore an ideal metric for adjusting chronic dosing in the obese.