Hypomagnesemia is seen in 11% of the normal hospital population and in up to 65% of severely ill patients, and can have many causes. The clinical picture may comprise the signs of Chvostek and Trousseau, paraesthesias, tremor and convulsions, although asymptomatic hypomagnesemia is seen most often. Hypomagnesemia can be treated by parenteral or oral administration of magnesium, guided by the serum magnesium level. Parenteral magnesium suppletion is indicated if the concentration is below 0.5 mmol/l or in the presence of symptoms of hypomagnesemia. In patients with magnesium concentrations between 0.5 and 0.7 mmol/l and a deficient diet or malabsorption, prolonged therapy is sometimes necessary. In such cases, oral Mg-containing antacids in a normal dosage regimen may be a good choice, but clinical proof is still lacking. In case of renal insufficiency or constipation the magnesium dose should be lowered, while in hypophosphatemia oral antacids are contra-indicated.