Despite widespread uptake of bariatric procedures for severe obesity, changes in pharmacodynamics after surgery are poorly understood. We report an epileptic patient who had a seizure following gastric bypass, although he had been asymptomatic for 30 years and without any change in his treatment. Phenytoin levels were undetectable despite a high dose. Drugs with a narrow therapeutic range such as phenytoin should be prescribed with caution after bariatric surgery.