Severe hypomagnesemia and secondary hypocalcemia associated with vonoprazan in a patient with type 2 diabetes

Hypomagnesemia is a well-known complication of long-term proton pump inhibitor usage; the stronger acid-suppressive effect of potassium-competitive acid blockers (P-CABs) may increase the risk of hypomagnesemia to a similar or greater extent. However, reports of P-CAB-associated hypomagnesemia are scarce. A 78-year-old Japanese man with a history of type 2 diabetes and myocardial infarction, who was receiving low-dose aspirin and long-term vonoprazan, presented with epigastric discomfort. Laboratory testing revealed significant hypocalcemia, hypomagnesemia, and low intact parathyroid hormone levels, which were consistent with functional hypoparathyroidism. Electrocardiogram (ECG) showed QTc prolongation and frequent premature ventricular contractions. Urinary magnesium excretion was low, suggesting impaired intestinal magnesium absorption rather than renal loss. Vonoprazan was discontinued, and magnesium and calcium supplementation was initiated. Electrolyte abnormalities normalized by day 3, and ECG changes subsequently resolved. No recurrence occurred after switching to an H₂-receptor antagonist. This case demonstrates severe hypomagnesemia and secondary hypocalcemia associated with long-term use of vonoprazan. P-CABs are often used with low-dose aspirin for ulcer prophylaxis against atherosclerotic diseases in patients with diabetes. Clinicians should be aware of hypomagnesemia as an adverse effect of P-CABs and consider periodic monitoring of serum magnesium levels, particularly in high-risk patients, such as those with type 2 diabetes.