The risk of hypercalcaemia should be considered in patients taking thiazide diuretics since these drugs can reduce calcium excretion. Calcium may reduce the absorption of thyroxine, bisphosphonates, sodium fluoride, quinolone and tetracycline antibiotics or iron. Concomitant treatment with phenytoin or barbiturates can decrease the effect of vitamin D. Concomitant use of glucocorticoids can decrease the effect of vitamin D. The effects of digitalis and other cardiac glycosides may be enhanced with the oral administration of calcium and Vitamin D.