Magnesium oxide (MgO) is a safe cathartic that rarely causes hypermagnesemia. Here, we report a schizophrenic patient case about loss of consciousness induced by hypermagnesemia due to MgO use. A 60-year-old man with schizophrenia had been taking antipsychotics and MgO for long time. After he had been constipated for 13 days, he defecated, then lost consciousness and developed hypotension. Hypermagnesemia (serum Mg level 20.2 mg/dL) was thought to be the cause of the circulatory collapse and loss of consciousness. Hemodialysis and intravenous calcium gluconate administration were performed. With his serum Mg level decrease, his condition improved. Mg is absorbed in the small intestine and excreted renally. Renal dysfunction and bowel hypomotility are the risk factors for hypermagnesemia. One of the side-effects of antipsychotics is ileus. If a patient takes both, Mg level could be high. Serum Mg level should be measured when a patient takes MgO and antipsychotics.