Utilization pattern of metamizole in northern Sweden and risk estimates of agranulocytosis.

OBJECTIVE: This study was carried out in order to investigate the utilization pattern of metamizole to better estimate the quantitative risk of agranulocytosis since a cluster of such cases have been observed in Sweden.
METHODS: Cases of agranulocytosis submitted to the Swedish Adverse Drug Reactions Advisory Committee (SADRAC) between 1996 and 1999 were identified. Based on the utilization pattern of metamizole in inpatients at three hospitals and in outpatients in two counties in northern Sweden risk estimates of agranulocytosis during metamizole treatment were estimated. The utilization of metamizole was investigated by scanning 3567 case records at 10 hospital departments as well as stored prescriptions at six pharmacies during a 3-month study period.
RESULTS: Ten cases of agranulocytosis during treatment with metamizole have been reported to SADRAC over the period 1996 to 1999. During the 3-month study period metamizole was prescribed to 666 (19%) inpatients. Of these, approximately 96% received the drug for less than 1 week, 7.2% had used the drug previously. At the participating pharmacies 112 metamizole prescriptions for outpatients were found. The drug was prescribed in 34% for less than 1 week, in 28% for 7-15 days, and in 38% for more than 15 days. The mean prescribed daily dose was 2.7 g. Given certain assumptions including the actual amounts prescribed the calculated risks of agranulocytosis would be approximately one out of every 31,000 metamizole-treated inpatients and one of every 1400 metamizole-treated outpatients.
CONCLUSION: This study indicates that in most inpatients the use of metamizole in northern Sweden was within the approved indications for the drug. However, a considerable number of outpatients received the drug for a longer time than recommended and this may carry an increased risk for developing agranulocytosis.