Hypomagnesaemia is relatively common, with an estimated prevalence in the general population ranging from 2·5% to 15%. It may result from inadequate magnesium intake, increased gastrointestinal or renal loss or redistribution from extracellular to intracellular space. Drug-induced hypomagnesaemia, particularly related to proton pump inhibitor (PPI) therapy, is being increasingly recognized. Most patients with hypomagnesaemia are asymptomatic; symptomatic magnesium depletion is often associated with multiple other biochemical abnormalities, including hypokalaemia, hypocalcaemia and metabolic acidosis. Manifestations of symptomatic hypomagnesaemia most often involve neuromuscular, cardiovascular and metabolic features. Patients with symptomatic hypomagnesaemia should be treated with intravenous magnesium, reserving oral replacement for asymptomatic patients.