The efficacy of magnesium oxide(MgO)when taken daily as a laxative for hypomagnesemia and renal dysfunction due to cisplatin(CDDP)administration is not clear. It is known that the efficacy of MgO is suppressed when used in combination with antacids, such as proton pump inhibitors. Therefore, we conducted a retrospective analysis of the effects of MgO and antacid administration on serum Mg levels and renal function(estimated glomerular filtration rate: eGFR)in CDDP-treated patients. In the analysis of up to 6 cycles of CDDP administration, the serum Mg levels and eGFR of CDDP-treated patients (n=614)were significantly reduced(p<0.001 and p=0.002, respectively, ANOVA). Patients who used MgO had higher serum Mg levels than patients who did not use MgO(p<0.001, ANOVA). However, no effect of MgO administration was observed on eGFR(p=0.832, ANOVA). There was no effect of antacid combination on serum Mg levels and eGFR in patients receiving MgO. MgO use may have contributed to a reduction in hypomagnesemia. However, it was considered that the decrease in eGFR could not be suppressed as the improvement in hypomagnesemia was slight. Intravenous Mg supplementation is required when CDDP is administered. Furthermore, it is expected that oral Mg supplementation will improve Mg absorption.