Although magnesium oxide is widely used as a laxative, alterations in serum magnesium concentrations among patients taking daily magnesium oxide have not been clarified. The present retrospective, cross-sectional study investigated the risk factors for hypermagnesemia in patients taking daily oral magnesium oxide. Of 2,176 patients administered daily magnesium oxide, 193 (8.9%) underwent assays of serum magnesium concentrations and were evaluated. High serum magnesium concentration and hypermagnesemia were defined as serum magnesium concentrations ≥2.5 mg/dl and ≥3.0 mg/dl, respectively. Of the 193 patients taking daily magnesium oxide, 32 (16.6%) had high serum magnesium concentration and 10 (5.2%) had hypermagnesemia. Factors associated with hypermagnesemia included chronic kidney disease (CKD) grade 4 (p = 0.014) and magnesium oxide dosage (p = 0.009). Factors associated with high serum magnesium concentration included magnesium oxide dosage >1,000 mg/day (p = 0.004), CKD grades 4 (p = 0.000) and concomitant use of stimulant laxatives (p = 0.035). Age, however, was not associated with hypermagnesemia or high serum magnesium concentration. In conclusion, renal function and magnesium oxide dosage, but not age, were associated with hypermagnesemia and high serum magnesium concentration in patients with functional constipation taking daily magnesium oxide.