May cause/aggravate elevated serum Ca levels w/ Ca-containing drugs. Disturbed absorption of drugs that form hardly soluble compd w/ Ca (tetracyclines, quinolones eg, ciprofloxacin & norfloxacin as well as drugs containing Fe, fluoride or estramustine). Increased risk for arrhythmia w/ cardioactive glycosides. Reduce dose if necessary when used w/ Al hydroxide.