Activated charcoal's adsorptive capacity, and therefore potential efficacy, is generally related to its surface area. In our study, the efficacy of two activated charcoal preparations, Actidose-Aqua 1,500 m2/g and Super Char, 3,000 m2/g, were compared on the basis of their ability to inhibit aspirin absorption. Twelve healthy male subjects fasted for eight hours before and four hours after a 20 mg/kg oral dose of aspirin. One hour after aspirin dosing, each subject received either no charcoal, 25 g Actidose-Aqua, or 25 g Super Char in a randomized crossover design. Each aspirin dose was separated from the previous dose by at least seven days. Total urine volumes were collected over 12-hour intervals, beginning 12 hours before the aspirin dose and continuing for 72 hours after dosing. Urine salicylate concentration was measured with a colorimetric assay. The fraction of aspirin dose recovered in the urine was 0.96 +/- 0.13, 0.78 +/- 0.18, and 0.50 +/- 0.20 for the control, Actidose-Aqua, and Super Char treatment phases, respectively. In vitro, Super Char was found to bind more salicylic acid than Actidose-Aqua at pH 8.1. We conclude that both activated charcoal preparations significantly reduced the gastrointestinal absorption of aspirin (P less than .05) and that Super Char was significantly more effective than Actidose-Aqua in reducing the gastrointestinal absorption of aspirin (P less than .01).