The possibility of hypercalcemia should be considered in patients using thiazide diuretics, as these drugs can decrease urinary calcium excretion. Hypocalcemia should be avoided in patients receiving digitalis therapy. Certain foods (such as those containing oxalic acid, phosphate, or phytic acid) may reduce calcium absorption. Concurrent use of phenytoin or barbiturates may decrease the effectiveness of Vitamin D3 due to increased metabolic activation. The effect of Vitamin D3 may also be reduced when used with glucocorticoids. Calcium combined with Vitamin D3 may diminish the effects of digitalis and other cardiac glycosides; therefore, careful medical supervision is required, including ECG monitoring if necessary. Calcium salts may interfere with the absorption of thyroxine, bisphosphonates, sodium fluoride, quinolones, tetracycline antibiotics, and iron. It is recommended to maintain at least a four-hour interval before taking calcium with these medications.