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According to epidemiological studies, constipation has a negative effect on life expectancy, necessitating appropriate treatment. According to the Pharmaceuticals and Medical Devices Agency (PMDA), patients who have been taking magnesium oxide (MgO) for constipation over a prolonged period, especially those with impaired renal function and older individuals, are at high risk of hypermagnesemia. Therefore, serum Mg levels, which are often not checked in clinical practice, should be monitored in these patients. Thus, to predict elevated serum Mg levels and prevent the development of hypermagnesemia, we aimed to identify the risk factors of hypermagnesemia, especially in the older population. Our study included patients who were prescribed MgO at our hospital between January 1, 2014, and March 31, 2016. Patients who did not meet the inclusion criteria were excluded and matched to adjust for background factors; finally, 35 patients in the hypermagnesemia arm and 140 patients in the non-hypermagnesemia arm were included in the analysis. Multivariate analysis identified estimated creatinine clearance (eCcr) ≤ 28.2 mL/min as a statistically significant risk factor. In addition, MgO dose ≥ 900 mg/day was identified as a risk factor for clinical consideration, although not statistically significant. Furthermore, the incidence of hypermagnesemia was shown to increase to 11.6% for those with MgO dose ≥ 900 mg/day, 27.0% for those with eCcr ≤ 28.2 mL/min, and 53.1% for those with both. Hypermagnesemia may occur in older patients with eCcr ≤ 28.2 mL/min who take more than 900 mg/day of MgO.