AbstractAimAn association between constipation and psychiatric disorders has been suggested. While magnesium oxide (MgO) is frequently administered in Japan to treat constipation, there is a risk of developing hypermagnesemia. We aimed to investigate the characteristics of hypermagnesemia in patients admitted to psychiatric hospitals.MethodsAmong all patients admitted to Ainohanazono Hospital between January 2020 and December 2023 who had undergone ≥1 blood test, we retrospectively investigated the risk of developing hypermagnesemia in those with or without MgO, patient characteristics associated with this risk, and characteristics that determined the MgO dosage.ResultsOf 1120 patients, 532 (48%) were prescribed MgO. The risk of developing hypermagnesemia was 2.76 times higher for patients prescribed MgO than for those not prescribed MgO. Patient characteristics associated with the risk of developing hypermagnesemia were daily MgO dose (hazards ratio [HR] 2.66), serum creatinine level (HR 1.75), and female biological sex (HR 1.44). Regarding patient characteristics that determined the prescribed MgO dosage, concomitantly administered antipsychotic and anticholinergic drugs were associated with an increase in the amount prescribed (odds ratio [OR] 1.12 and OR 1.06, respectively). However, long‐term treatment duration (OR 0.91) and blood urea nitrogen levels (OR 0.95) were associated with a decrease in the dosage.ConclusionWe clarified the risk of MgO‐related hypermagnesemia, particularly for patients undergoing long‐term (>1 year) MgO administration. Antipsychotic and anticholinergic drug dosages slightly affected the increase in the MgO dosage. Regular blood sampling is needed to prevent hypermagnesemia, with a switch to a non‐MgO laxative when renal function is impaired.