Metamizole inhibits arachidonic acid-induced platelet aggregation after surgery and impairs the effect of aspirin in hospitalized patients.

PURPOSE: The inhibitory effect of metamizole on platelet aggregation is known for several years, but most studies were conducted in healthy volunteers with contradictory results. Recent studies have shown an inhibitory effect of metamizole on acetylsalicylic acid (ASA)-induced platelet aggregation. We aimed to investigate the effect of metamizole on platelet aggregation after an elective surgery and the effect of metamizole on ASA-induced platelet aggregation in hospitalized patients.

METHODS: We performed platelet aggregation analysis after induction with ADP, arachidonic acid (AA), epinephrine, and collagen in 37 patients prior to an elective visceral or thoracic surgery and on postoperative day (POD) 1 and POD 3 1-2 h and 5-6 h after metamizole. In another cohort of 10 hospitalized patients receiving the combination of metamizole and ASA for more than 7 days, AA-induced platelet aggregation was analyzed in the morning prior to the intake of both drugs.

RESULTS: Metamizole induced a strong inhibitory effect on AA-induced platelet aggregation at all time points being detectable up to 41 h in some patients. Besides a less pronounced effect on collagen-induced platelet aggregation on POD 3 1-2 h after metamizole, all other inductors showed no effect. In 4 out of 10 hospitalized patients, no ASA-induced inhibition of platelet aggregation was detectable without correlation to sequence of administration.

CONCLUSIONS: The reason why some patients have a long-lasting inhibitory effect of metamizole on COX-induced platelet aggregation that might interfere with ASA should be investigated in a larger cohort of patients.