<p>The risk of hypercalcemia should be considered in patients taking thiazide diuretics since these drugs can reduce urinary calcium excretion. Hypercalcemia must be avoided in digitalised patients. Certain foods (e.g. those containing oxalic acid, phosphate or phytinic acid) may reduce the absorption of calcium. Concomitant therapy with phenytoin or barbiturates can decrease the effect of vitamin-D because of metabolic activation. The effect of digitalis and other cardiac glycosides may be accentuated with the oral administration of calcium combined with vitamin-D. Calcium salts may reduce the absorption of thyroxine, bisphosphonates, sodium fluoride, quinolone or tetracycline antibiotics or iron. It is advisable to allow a minimum period of 4 hours before taking the<br />
calcium.</p>