Effect of purgatives on antidotal efficacy of oral activated charcoal

The effects of purgatives on the antidotal efficacy of oral activated charcoal were studied in seven volunteer subjects. The volunteer subjects were given 1000 mg of aspirin, 100 mg of atenolol and 50 mg of phenylpropanolamine with 100 ml of water on an empty stomach and were assigned randomly to the following treatment groups: after 5 min 150 ml of water, after 5 min 25 g of charcoal, after 5 min charcoal orally with 20 mg of metoclopramide rectally, followed by 10 mg of bisacodyl rectally 3 h afterwards, after 5 min charcoal with 250 ml of magnesium citrate USP and after 60 min charcoal with metoclopramide followed by bisacodyl 3 h thereafter. The plasma concentrations (0-24 h) and the cumulative urinary excretion (0-72 h) of salicylates, atenolol and phenylpropanolamine were measured. Both magnesium citrate and metoclopramide combined with bisacodyl hastened the gastrointestinal transit but magnesium citrate was more effective. Charcoal alone reduced the absorption of aspirin and phenylpropanolamine by about 50% and that of atenolol by about 95%. The purgatives did not modify significantly the efficacy of charcoal. When the antidotal treatment was delayed by 60 min its efficacy was reduced to some extent, possibly depending on the pharmaceutical formulation of the test drugs. The present results do not support the routine use of purgatives in combination with activated charcoal. In some instances, however, their use may promote the evacuation of, for example, depot formulations from the gastrointestinal tract and thus have a beneficial effect together with activated charcoal in reducing absorption.