[Skip to Content]
Access to paid content on this site is currently suspended due to excessive activity being detected from your IP address Please contact the publisher to request reinstatement.
[Skip to Content Landing]
May 1985

Phenytoin Disposition in Obesity: Determination of Loading Dose

Arch Neurol. 1985;42(5):468-471. doi:10.1001/archneur.1985.04060050066010

• Fourteen obese subjects (mean body weight, 124 kg; percent of ideal body weight [IBW], 178%) and ten control subjects of normal body habitus (mean body weight, 67 kg; 92% IBW) received 300 mg of phenytoin sodium by tenminute intravenous infusion. Obese subjects compared with controls had prolonged phenytoin elimination half-life (19.9 v 12.0 hours). Total metabolic clearance of phenytoin was greater in obese than in control groups, although the difference was not significant (59 v 39 mL/ min). Phenytoin half-life, inversely proportional to clearance and directly proportional to volume of distribution (Vd), was prolonged in obesity mainly as a result of the increase in Vd in obese subjects (84 v 40 L). Phenytoin loading dose should be calculated on the basis of IBW plus the product of 1.33 times the excess weight over IBW. Very obese individuals will require large absolute loading doses of phenytoin to rapidly achieve therapeutic drug concentrations.