May cause or aggravate elevated serum Ca levels w/ Ca-containing drugs. Risk of disturbed absorption w/ drugs forming hardly soluble compd w/ Ca (eg, tetracyclines, quinolones (eg, ciprofloxacin & norfloxacin) as well as drugs containing Fe, fluoride or estramustine). Increased risk of arrhythmia w/ cardioactive glycosides in the presence of hypercalcemia. Al HCl. Monitor decrease of serum phosphate levels.