Dipyrone (INN, metamizol) is a common analgesic used worldwide. Its widespread prescription or over-the-counter use in many countries (e.g., Brazil, Israel, Mexico, Russia, Spain) requires insight into its mode of action. This study therefore addressed the impact of its metabolites 4-methyl-amino-antipyrine (MAA) and 4-amino-antipyrine (AA) on peripheral cyclooxygenases (COX). Pharmacokinetics of metabolites and ex vivo COX inhibition were assessed in five volunteers receiving dipyrone at single oral doses of 500 or 1000 mg. Coagulation-induced thromboxane B2 formation and lipopolysaccharide-induced prostaglandin E2 synthesis were measured in vitro and ex vivo in human whole blood as indices of COX-1 and COX-2 activity. In vitro, metabolites elicited no substantial COX-1/COX-2 selectivity with MAA (IC50=2.55 micromol/L for COX-1; IC50=4.65 micromol/L for COX-2), being approximately 8.2- or 9-fold more potent than AA. After administration of dipyrone, MAA plasma concentrations remained above the IC50 values for each isoform for at least 8 h (500 mg) and 12 h (1000 mg) postdose. COX inhibition correlated with MAA plasma levels (ex vivo IC50 values of 1.03 micromol/L [COX-1] and 0.87 micromol/L [COX-2]). By contrast, plasma peak concentrations of AA after the 1000 mg dose were 2.8- and 6.5-fold below its IC50 values for COX-1 and COX-2, respectively. Maximal inhibitions of COX-1 and COX-2 were 94% and 87% (500 mg), 97% and 94% (1000 mg). Taken together, dipyrone elicits a substantial and virtually equipotent inhibition of COX isoforms via MAA. Given the profound COX-2 suppression by dipyrone, which was considerably above COX-2 inhibition by single analgesic doses of celecoxib and rofecoxib, a significant portion of its analgesic action may be ascribed to peripheral mechanisms. In view of the observed COX-1 suppression, physicochemical factors (lack of acidity) rather than differential COX-1 inhibition may be responsible for dipyrone's favorable gastrointestinal tolerability compared with acidic COX inhibitors.