Enteric-coated charcoal as a means of blinding studies comparing bismuth and H2-receptor antagonists

Clinical trials comparing H2-receptor antagonists and bismuth-containing regimens in ulcer healing, ulcer relapse, or dyspepsia have not been double-blind, because bismuth darkens stools. This study was designed to determine whether enteric-coated charcoal could darken stools of subjects taking an H2-receptor antagonist without interfering with absorption of the drug. Twelve healthy volunteers participated in a randomized, double-blind, crossover trial of oral ranitidine (300 mg at bedtime) alone, ranitidine plus bismuth (30 ml Pepto-Bismol four times daily), and ranitidine plus enteric-coated charcoal (four tablets daily). Mean stool colors both with ranitidine plus charcoal and with ranitidine plus bismuth were significantly darker than with ranitidine alone, but were not significantly different from each other. Blood levels of ranitidine during the 10 h after its administration did not differ significantly among the three regimens. We conclude that enteric-coated charcoal can be used to blind clinical trials comparing an H2-receptor antagonist (e.g., ranitidine) and a bismuth regimen.