Influence of subsalicylate bismuth on absorption of doxycycline

The influence of a typical 60-mL dose of subsalicylate bismuth (Pepto-Bismol) on the absorption of 200 mg of orally administered doxycycline hyclate was studied. Bioavailability of doxycycline was significantly reduced by 37% and 51%, respectively, when subsalicylate bismuth was given simultaneously and as a multiple-dose regimen before doxycycline. Peak serum concentrations of doxycycline were significantly decreased when subsalicylate bismuth was given two hours before doxycycline but not when given two hours after doxycycline. Subsalicylate bismuth should not be taken when doxycycline is used for therapeutic purposes, and we suggest travelers should not take the agents together in an effort to prevent diarrhea.