BACKGROUND Dysmenorrhea produces painful abdominal cramps that can disrupt the personal lives and productivity of women. OBJECTIVE The aim of this study was to compare the analgesic efficacy, including onset and duration of pain relief, peak effect, and total effect, and tolerability of ibuprofen arginate with those of conventional ibuprofen in patients with moderate to severe pain associated with primary dysmenorrhea. METHODS Patients were administered a single dose of ibuprofen arginate (200 or 400 mg), conventional ibuprofen (200 or 400 mg), or placebo during each of 5 menstrual cycles in a single-center, double-blind, randomized, double-dummy, 5-cycle, crossover study. Patients recorded their pain intensity and pain relief at regularly scheduled intervals (10, 20, 30, 40, 50, 60, and 90 minutes and 2, 3, 4, 5, and 6 hours) after taking the study medication, and all study observations were recorded in a patient diary. Pain intensity was rated using the following 4-point categoric rating scale: 0 = none, 1 = mild, 2 = moderate, and 3 = severe. Pain relief was rated on a 5-point scale as 0 = none, 1 = a little, 2 = some, 3 = a lot, and 4 = complete relief. Tolerability of ibuprofen arginate was based on a comparison of the incidence of spontaneously reported adverse events in each of the treatment groups. RESULTS One hundred four patients entered the study. Of these, 81.7% were white; the mean (SD) age was 27.5 (5.0) years. A total of 65.4% of patients reported moderate pain from dysmenorrhea, and the remaining 34.6% reported severe pain; 20.2% of patients did not complete the study. The median time to achieve meaningful pain relief was ∼30 minutes faster with ibuprofen arginate 400 mg than with either dose of conventional ibuprofen. Tolerability was similar across all treatments. CONCLUSIONS In this study population of patients experiencing acute pain as a result of primary dysmenorrhea, ibuprofen arginate was associated with effective, tolerable analgesia and a more rapid onset of action than conventional ibuprofen. The faster onset of analgesia may have a role in clinical practice in treating women with dysmenorrhea. A faster onset of action may be important to women whose personal relationships, productivity, or ability to sleep is being adversely affected by pain.