Side effects of antacid therapy are dose dependant and compound related. High dose antacid intake may lead to fluid retention in the body depending on the sodium content of the different antacid preparations. Sodium bicarbonate ingestion provokes metabolic alkalosis and alkaliuria, the "nonsystemic calcium and magnesium containing antacids" cause these changes too, but to a lower degree. Urinary pH elevation favours the precipitation of calcium and magnesium salts, predisposing to renal stone formation. In patients with renal insufficiency the calcium and magnesium absorption may lead rapidly to toxic serum concentrations. Calcium and magnesium containing acids may provoke an acid rebound, which is clinically not relevant following magnesium-hydroxide-ingestion. Phosphorus depletion is an important side effect of aluminum hydroxide intake. The phosphorus depletion syndrome combined with skeletal demineralisation and osteomalacia may occur. As well as calcium and magnesium cations the tribasic aluminum will be absorbed from the gut in small amounts. In patients with renal insufficiency aluminum deposition in the brain grey matter following Al(OH)3 ingestion will occur and seems to be a co-factor for the development of a dialysis encephalopathy syndrome. The clinical relevance of aluminum absorption from gut in patients with normal renal function is unknown until now.