Because treatment with antiepileptic drugs (AEDs) is often for years or lifelong, physicians should be aware of the metabolic changes that can be associated with AED use and the potential effects of these changes during long-term therapy. Alterations of bone metabolism leading to decreased bone mineral density, associated particularly but not exclusively with the hepatic enzyme-inducing AEDs, can worsen the risk for fractures, which is already increased in patients with epilepsy by factors such as seizure-related falls and trauma. Some AEDs are associated with weight gain, an effect that is not only distressing to many patients but may be sufficient to increase the risk for cardiovascular disease and other disorders associated with excessive body weight. The carbonic anhydrase-inhibiting properties of some AEDs can lead to metabolic acidosis. The AEDs that inhibit carbonic anhydrase are also associated with an increase in risk for renal stones, as is the ketogenic diet. Awareness of the potential metabolic disturbances associated with AED use is particularly important because many of them are subtle and may take years to become clinically apparent.