Effect of gastric pH on antidotal efficacy of activated charcoal in man

Environmental pH is important for the adsorption capacity of activated charcoal: in our experiments the unadsorbed fractions of aspirin and disopyramide were increased by 10-20-fold as the pH was altered from 1.2 to 7.0 or vice versa. In order to study the effect of pH in vivo, six subjects were given 500 mg aspirin, 200 mg disopyramide and 200 mg tolfenamic acid on an empty stomach with 20 ml of 8.5% magnesium hydroxide or without it. A small dose of charcoal, 2.5 g, administered immediately after the drugs, reduced the absorption of aspirin by 30-40%, whereas the absorption of disopyramide and tolfenamic acid was reduced by 70-80%. The inhibition of absorption was irrespective of whether the drugs were taken with the antacid or without it. Thus, in vivo other factors than the gastric pH must be more important in controlling the adsorption to activated charcoal. Accordingly, the simultaneous administration of antacids cannot be recommended to enhance the adsorptive capacity of charcoal in humans.