May cause or aggravate elevated serum Ca levels w/ Ca-containing drugs. Take drugs that form hard soluble compd w/ Ca eg, tetracyclines, quinolones (eg, ciprofloxacin & norfloxacin), drugs containing Fe, fluoride or estramustine) w/ interval of at least 2 hr. Increased risk of arrhythmia w/ cardioactive glycosides. Reduce dose in case of Al hydroxide administration.