Toxicity may include interstitial pulmonary fibrosis ("busulfan lung"), hyperpigmentation, seizures, hepatic (veno-occlusive disease) (VOD) or sinusoidal obstruction syndrome (SOS), emesis, and wasting syndrome. Busulfan also induces thrombocytopenia, a condition of lowered blood platelet count and activity, and sometimes medullary aplasia. Seizures and VOD are serious concerns with busulfan therapy and prophylaxis is often utilized to avoid these effects. Hepatic VOD is a dose-limiting toxicity. Symptoms of VOD include weight gain, elevated bilirubin, painful hepatomegaly, and edema. The reason busulfan causes VOD is mostly unknown and can be deadly. Ursodiol may be considered for prophylaxis of veno-occlusive disease.