BACKGROUND Although hypomagnesemia is common in type 2 diabetes, clinical presentations with severe hypomagnesemia are rare. A number of oral blood glucose-lowering medications can reduce serum magnesium concentrations, and several severe cases have been reported in the presence of marked glucagon-like peptide-1 receptor agonist (GLP-1RA)-associated gastrointestinal adverse effects. In the present case, an acute presentation with severe hypomagnesemia was likely due to polypharmacy including semaglutide, albeit with a delayed relationship to discontinuation of this GLP-1RA, due to nausea and vomiting. CASE REPORT A 73-year-old woman with type 2 diabetes treated with several oral medications known to reduce serum magnesium (metformin, gliclazide, sitagliptin, esomeprazole) presented after an unwitnessed collapse at home without premonitory symptoms. She had discontinued low-dose semaglutide (0.25 mg subcutaneous weekly) 2 weeks beforehand, and her gastrointestinal adverse effects had largely resolved. She was found to have an undetectable serum magnesium (<0.3 mmol/L) and hypocalcemia. She responded to electrolyte replacement and was discharged well 2 days later. Three weeks after discharge, her serum magnesium and calcium concentrations were within the reference range, on regular oral supplements of both minerals. She spontaneously reported her longstanding muscle cramps had resolved after discharge. Her clinical features and course suggested she had chronic unrecognized hypomagnesemia associated with polypharmacy that progressed to a clinically severe level, with a likely contribution from recent antecedent semaglutide use. CONCLUSIONS Periodic monitoring of serum magnesium concentrations in at-risk individuals with type 2 diabetes is recommended, since the clinical presentation of severe hypomagnesemia can be sudden and without indicative warning symptoms.