Activated charcoal adsorption of diphenhydramine.

The adsorption of the antihistamine diphenhydramine (D) by activated charcoal (AC) was assessed in vitro and in vivo in six healthy volunteers in order to assess the utility of AC as an adjunct in the treatment of antihistamine overdose. Results of greater than 85% adsorption with AC:D weight ratios of 10:1 or greater in the in vitro studies led to a three-way crossover trial in six volunteers. Fifty mg D was administered alone (control), with 50 g AC within 5 minutes of D (D + C5) and with 50 g AC 60 minutes after D (D + C60). Mean reductions in peak serum D concentrations of 94.8% (D + C5) and 12.3% (D + C60), AUC 0-24 h of 96.9% (D + C5) and 20.4% (D + C60), and AUC 0-omega of greater than 90% (D + C5) and 24.0% (D + C60) were noted, although only the comparison of D with D + C5 reached statistical significance due to wide interpatient variation. A positive correlation of time to peak concentration of drug in serum in the control group with percentage reduction in AUC in the D + C60 group further indicate the potential for positive therapeutic benefit of AC in antihistamine overdose. This is because lag time and time to peak concentration may be excessively prolonged in overdose due to anticholinergic-induced alterations in G.I. motility. Maximal and multiple doses of AC are recommended for adjunctive treatment of antihistamine overdose.