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Study objective: We evaluate the analgesic and side effects of adding cyclobenzaprine to ibuprofen in emergency department patients with acute myofascial strain. Methods: A randomized, prospective, double-blind study was conducted at an urban teaching ED with an annual census of 44,000. One hundred two patients aged 18 to 70 years with acute myofascial strain caused by minor trauma within the prior 48 hours were included, and 77 patients completed the protocol. Each patient received a single dose of 800 mg of ibuprofen in the ED and a vial of 6 capsules containing 800 mg of ibuprofen to take every 8 hours as needed after discharge from the ED. In addition, 51 patients received a single dose of 10 mg of cyclobenzaprine and a vial of 6 capsules containing 10 mg of cyclobenzaprine to take every 8 hours as needed after discharge from the ED; the remaining 51 patients received an identically labeled placebo capsule and vial of placebo capsules to take every 8 hours as needed after discharge from the ED. Patients rated the intensity of their pain on a 100-mm visual analog scale (VAS) at baseline; 30, 60, 90, 120, and 180 minutes; and 24 and 48 hours after treatment. Telephone follow-up was obtained at 24 and 48 hours, and side effects were elicited at 24 and 48 hours by means of open-ended questioning. Results: The patients in each group were similar with regard to diagnosis and baseline pain score. The number of patients who did not complete the protocol and the number of those who required additional analgesia were similar in both groups. Over the 48 hours of the protocol, the mean VAS score for the combination group decreased from 60.4 to 35.6, and the mean VAS score for the ibuprofen alone group decreased from 62.2 to 35.4. The mean VAS scores between groups across time was not statistically significant (P=.962, repeated-measures analysis of variance). At both 24 and 48 hours, central nervous system side effects were reported more frequently in the patients receiving cyclobenzaprine (16 [42%] versus 7 [18%] at 24 hours and 15 [39%] versus 5 [13%] at 48 hours, respectively). Conclusion: In ED patients with acute myofascial strain, the addition of cyclobenzaprine to ibuprofen does not improve analgesia but is associated with a greater prevalence of central nervous system side effects.