Decreased tetracycline bioavailability caused by a bismuth subsalicylate antidiarrheal mixture

Oral coadministration of a single 250-mg tetracycline capsule and 60 ml of a bismuth subsalicylate antidiarrheal mixture reduced tetracycline absorption by 34% without appearing to perturb its absorption or disposition rate. A pronounced increase in intersubject tetracycline absorption variability also was noted. Apparently, the reduction in tetracycline bioavailability previously reported with a kaolin-pectin suspension is not peculiar to kaolin-pectin but can be expected with almost any antidiarrheal whose mechanism of action is adsorptive in nature.