Decreased bioavailability of propranolol due to interactions with adsorbent antacids and antidiarrhoeal mixtures

The interaction of propranolol (50–160 mg/100 ml) with recommended doses of magnesium trisilicate, kaolin-pectin and bismuth subsalicylate suspensions was studied. Magnesium trisilicate and bismuth subsalicylate were found to adsorb propranolol with a limiting adsorptive capacity of 112 and 97 mg/g, respectively. Kaolin-pectin suspension adsorbed practically all of the drug. The effect of these interactions on the bioavailability of propranolol in rats was determined. The extent, but not the rate. of propranolol absorption was decreased on concomitant administration of the adsorbents. Peak propranolol plasma concentrations decreased by 292 5% and a parallel decrease of 35–44% in AUC values was observed. These interactions suggest possible clinically important differences in propranolol bioavailability.