May develop hypermagnesemia w/ Mg-containing antacids in patients on chronic renal dialysis. May precipitate cardiac arrhythmias w/ digitalis glycosides. Atrial fibrillation may occur w/ verapamil when supplemental Ca is administered. Decreased conc & intestinal absorption (for Hello-D 2000 only) w/ cholestyramine. Synthetic & cationic enzymes may be inhibited by ketoconazole. Reduced absorption w/ mineral oil. Endogenous synthesis may be inhibited w/ phenytoin or phenobarb. May develop hypercalcemia w/ thiazide diuretics in hypoparathyroid patients.