As people age, their total body water decreases, their lean body mass is reduced, and their percentage of body fat increases. This increase in body fat expands the volume of distribution for lipophilic drugs and also decreases the volume of distribution for hydrophilic drugs.6 The result is that water-soluble medications have an elevated active serum concentration, and lipid-soluble agents, while they may have a decreased serum concentration, have a prolonged half-life.