Metamizole in the Treatment of Acute Postoperative Pain

Analyzed Studies

The systematic review included 15 randomized double-blind studies that examined the effects of a single dose of metamizole on moderate to severe postoperative pain during the 4–6 hours after administration. Metamizole was administered orally in doses of 500, 575, or 1000 mg, rectally in doses of 1000 mg, intramuscularly in doses of 2000 or 2500 mg, and intravenously in doses of 2500 mg. All studies had active control groups (ibuprofen, paracetamol, acetylsalicylic acid, flurbiprofen, ketoprofen, dexketoprofen, ketorolac, pethidine, tramadol, suprofen), and a total of 8 studies also compared metamizole with placebo. The methodological quality of these studies was assessed as medium.

A clinically significant outcome was defined as pain relief of at least half the maximum intensity (50% pain relief). Secondary outcomes included the number of participants requiring rescue medication, the median (or mean) time to use of rescue medication, and any adverse effects. Sufficient data for statistical analysis were available for only some outcomes and comparisons.

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Results

An oral dose of 500 mg metamizole provided at least 50% pain relief for 4–6 hours in 73% of patients (106/143; range 60–87%), while placebo provided this relief in 32% of patients (45/145; range 19–41%). The calculated NNT (number needed to treat) for metamizole to achieve at least 50% pain relief for 4–6 hours was 2.4 (1.9–3.1).

The proportion of patients who received 500 mg oral metamizole and needed rescue treatment was 7% (8/123; range 0–15%), whereas 34% of patients on placebo needed rescue analgesia (43/125; range 20–51%). The NNT for preventing the use of rescue medication was 3.6 (2.7–5.4).

It was also possible to compare with tramadol. The proportion of patients who experienced at least 50% pain relief for 4 hours after receiving 2.5 g intravenous metamizole was 70% (71/101; range 63–78%), whereas 100 mg intravenous tramadol relieved pain in 65% of patients (64/99; range 57–72%). No statistically significant difference was observed between the two analgesics. It was not possible to statistically evaluate and compare the need for rescue medication.

No blood dyscrasias or serious adverse events were reported in the studies included in the analysis. However, it is important to note that these were older studies with a relatively small number of participants, and the authors of the analysis themselves noted that the quality of adverse event reporting was not good. For this reason, no safety comparisons could be made.

Conclusion

Metamizole is an effective analgesic in the treatment of postoperative pain. Whether its potential benefits outweigh the possible risks, especially considering rare adverse effects, needs to be demonstrated through further, methodologically sound studies.

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Source:

Derry S., Faura C., Edwards J. et al. Single dose dipyrone for acute postoperative pain. Cochrane Database Syst Rev 2010; 9: CD003227, doi: 10.1002/14651858.CD003227.pub2.