Hypocholesterolemic drug therapy has become a major focus of cardiovascular medicine because recent evidence has demonstrated that optimization of serum lipid levels results in decreased atherosclerotic cardiovascular risk. Niacin, a B-complex vitamin, is capable of lowering total serum cholesterol and triglyceride levels while simultaneously raising serum high-density lipoprotein cholesterol levels. However, the use of niacin can be complicated by the development of hepatotoxicity, hyperuricemia, and modest hyperglycemia. A patient is described who had an excellent lipid response to niacin therapy but developed the previously undescribed complication of severe, life-threatening hyperglycemia. Therefore, despite its low cost and demonstrated efficacy, because of potential very toxic side effects, niacin therapy needs to be carefully monitored.