Al hydroxide: Constipation, nausea, vomiting, intestinal obstruction (large doses), phosphate depletion accompanied by increased resorption & urinary excretion of Ca w/ risk of renal rickets in the young or osteomalacia in older patients (excessive doses or in patients w/ low phosphate diets). Mg trisilicate: Diarrhoea, mucosal irritation & absorption of Mg, hypermagnesemia (excessive use).