Hematological safety of metamizole: retrospective analysis of WHO and Swiss spontaneous safety reports

Purpose: Since the 1970s, the use of metamizole is controversial due to the risk of agranulocytosis. The aim of this study was to analyze individual case safety reports (ICSRs) of metamizole-associated hematological adverse drug reactions (ADRs). Methods: International and Swiss metamizole-associated ICSR concerning selected hematological ADR were retrieved from VigiBase™, the World Health Organization Global Database of ICSR, and the Swiss Pharmacovigilance Database. We evaluated demographic data, co-medication, drug administration information, dose and duration of metamizole treatment, as well as the latency time of ADR, their course, and severity. The subgroup analysis of Swiss reports allowed us to analyze cases with fatal outcome more in depth and to estimate a rough minimal incidence rate. Results: A total of 1417 international and 77 Swiss reports were analyzed. Around 52% of the international and 33% of the Swiss metamizole-associated hematological ADR occurred within a latency time of ≤7days. More women were affected. The annual number of hematological reports and those with fatal outcome increased over the last years parallel to metamizole sales figures. In Switzerland, the minimal incidence rate of agranulocytosis was 0.46-1.63 cases per million person-days of use (2006-2012). Female sex, old age, pancytopenia, and co-medication with methotrexate were striking characteristics of the seven Swiss fatal cases. Conclusions: Metamizole-associated hematological ADR remain frequently reported. This is underscored by increasing annual reporting rates, which mainly reflect growing metamizole use. Early detection of myelotoxicity and avoidance of other myelotoxic substances such as methotrexate are important measures for preventing fatalities