Objective: Risk factors for hypermagnesemia due to magnesium oxide (MgO) include advanced age, decreased renal function, and long-term administration; however, no study has evaluated patients that present all of these factors. This study was aimed to evaluate the safety of long-term MgO administration in elderly patients with impaired renal function.Methods: We investigated changes in serum Mg in patients aged 65 years or older, who had been taking oral MgO and presenting a glomerular filtration rate (eGFR) ＜60 mL/min/1.73 m2 for 6 months or longer.Results: Thirty patients were surveyed. Their median age was 81 years (range, 68-92). No significant change in patient background was observed before and after initiating MgO administration, with no change in serum Mg detected. Furthermore, the oral dose of MgO was divided into groups taking ＜1,000 mg and ≥ 1,000 mg; no change in serum Mg was observed in either group. Based on renal function, the analysis was divided into a mildly decreased group (60＞ eGFR ≥ 45) and a moderately to severely decreased group (45＞ eGFR ≥ 15), and no change in serum Mg was observed in either group.Conclusions: We targeted elderly patients with renal dysfunction who were taking long-term MgO, a known risk factor for hypermagnesemia, indicating that MgO can be safely continued.