OBJECTIVE Nonsteroidal anti-inflammatory drugs (NSAID) are effective to treat migraine attacks. Lysine clonixinate (LC) and dipyrone (metamizol) have been proven effective to treat acute migraine. The aim of this study was to evaluate the efficacy and tolerability of the intravenous formulations of LC and dipyrone in the treatment of severe migraine attacks. METHODS Thirty patients (28 women, 2 men), aged 18 to 48 years with migraine according the International Headache Society (IHS) (2004) were studied. The patients were randomized into 2 groups when presenting to an emergency department with a severe migraine attack. The study was single-blind. Headache intensity, nausea, photophobia and side effects were evaluated at 0, 30, 60 and 90 minutes after the drug administration. Rectal indomethacin as rescue medication (RM) was available after 2 hours and its use compared between groups. RESULTS All patients completed the study. At 30 minutes, 0% of the dipyrone group 13% of the LC group were pain free (p=0.46). At 60 and 90 minutes, 2 (13%) and 5 (33%) patients from the dipyrone group and 11 (73%) and 13 (86.7%) patients from the LC group were pain free (p<0.001). At 60 minutes, significantly more patients from the LC group were nausea-free (p<0.001). Regarding photophobia, there were no differences between groups at 60 minutes (p=0.11). The use of RM at 2 hours did not differ among groups (p=0.50). Pain in the site of the injection was reported by more patients of the LC group compared to the dipyrone group (p<0.0001). CONCLUSIONS LC is significantly superior to dipyrone in treating severe migraine attacks. LC promotes significantly more burning at the site of the injection.