BACKGROUND: The clinical impact of hypomagnesemia induced by necitumumab plus gemcitabine and cisplatin (GCN) as a second-line or later therapy is unclear. OBJECTIVE: We aimed to evaluate the clinical characteristics and survival impact of hypomagnesemia induced by this therapy. DESIGN: This was a sub-analysis of the retrospective multicenter NINJA study. METHODS: Among the 93 patients enrolled in the NINJA study, this subanalysis included 75 patients with baseline serum magnesium concentrations. RESULTS: The incidence of grade ⩾2 hypomagnesemia was 18.0% in the patients with normal baseline serum magnesium concentrations and 42.8% in those with low concentrations ( CONCLUSION: GCN as a second-line or later therapy may be tolerable regardless of the patient's baseline serum magnesium concentration. On the other hand, early serum magnesium reduction with this therapy is associated with a poor prognosis. However, caution should be needed because our results lacked sufficient information for confounding variables other than those analyzed here that may influence the correlation between hypomagnesemia and survival.