A major active metabolite of metamizole inhibits A23187-induced PGE2 production in isolated mouse peritoneal macrophages at 0.1 mM increases the paw withdrawal threshold in a rat model of carrageenan-induced hyperalgesia at 160 Ug/paw reduces LPS-induced pyrexia in rats at 60 90 and 120 mg/kg a potential impurity found in commercial preparations of metamizole