Effect of activated charcoal on absorption and elimination of phenobarbitone, carbamazepine and phenylbutazone in man

Summary

The effect of activated charcoal, given as a water suspension, on the absorption and elimination of phenobarbitone 200 mg, carbamazepine 400 mg and phenylbutazone 200 mg, was studied in five healthy volunteers, using a randomized crossover design. Absorption of the drugs was almost completely prevented (more than 95%) when charcoal 50 g was ingested within five minutes of taking the drugs. When activated charcoal was taken one hour after the drugs, the mean inhibition of drug absorption was considerably less and the inhibition was greatly dependent on the individual rate of absorption. The half-life of phenobarbitone was markedly shortened from a control value of 110±23 h to 19.8±1.0 h (P<0.05), with a corresponding increase in plasma clearance from 4.6 ml/min to 23 ml/min, when activated charcoal 118 g, in five divided doses was given between 10 and 48 h after ingestion of the drug. The half-life of carbamazepine was shortened by 45% (P<0.05) and the reduction in the half-life of phenylbutazone (30%) by charcoal, too, was statistically significant. The only side effect from use of the charcoal suspension was constipation, which occured in three subjects after repeated doses, and which was easily treated with lactulose if required. Although activated charcoal should be given as soon as possible, even its delayed use may be indicated, due to the slow absorption often seen in acute intoxication. The use of multiple doses of charcoal appears to be indicated as an additional treatment of certain severe intoxications to prevent the release of drugs from charcoal, and to increase their rate of elimination if they are secreted into the gut with subsequent reabsorption.