High dose of antacid (Mylanta II) reduces bioavailability of ranitidine

To investigate the effect of antacid on the bioavailability and disposition of ranitidine six healthy volunteers were studied on two occasions one week apart. In the first study the received ranitidine 150 mg with 60 ml water, and in the second study they received ranitidine 150 mg plus 30 ml of an aluminium/magnesium hydroxide mixture (Mylanta II) and 30 ml water. Giving antacid reduced both the maximum plasma ranitidine concentration and the area under the curve by one-third; elimination of the drug was not changed. Thus giving a high dose of antacid significantly diminished the bioavailability of ranitidine.