Hypermagnesemia is an often-overlooked problem in acute medical care situations. Magnesium is used in several areas of medicine and for various reasons. It also is found in many seemingly innocuous cathartics and gastrointestinal medications. Magnesium replacement in enteral or parenteral feeding and magnesium deficiency is addressed often. Despite the necessity to consider magnesium in several situations, many physicians are relatively unfamiliar with the use of magnesium in their patients. Magnesium is often used in patients with renal insufficiency, which further increases the risk of hypermagnesemia. Fortunately, the evaluation and treatment of hypermagnesemia is relatively straightforward. Calcium reversal of life-threatening sequelae, diuresis or dialysis to augment elimination of magnesium, and supportive care until the acute manifestations of the disorder resolve are the mainstays of therapy for hypermagnesemia. Recognition of patients at risk for the disorder and careful consideration of magnesium therapy will allow for prevention or early diagnosis of the disorder.