Ibuprofen With or Without Acetaminophen for Low Back Pain

Inclusion Criteria: Present to emergency room primary for management of LBP, defined as pain originating between the lower border of the scapulae and the upper gluteal folds. Flank pain, that is pain originating from tissues lateral to the paraspinal muscles, will not be included. * Musculoskeletal etiology of low back. Patients with non-musculoskeletal etiologies such as urinary tract infection, ovarian cysts, or influenza like illness will be excluded. The primary clinical diagnosis, at the conclusion of the emergency room visit, must be a diagnosis consistent with non-traumatic, non-radicular, musculoskeletal low back pain. * Patient is to be discharged home. * Age 18-69 * Non-radicular pain. Patients will be excluded if the pain radiates below the gluteal folds in a radicular pattern. * Pain duration \<2 weeks (336 hours). * Prior to the acute attack of LBP, back pain cannot occur more frequently than once per month. * Non-traumatic LBP: no substantial and direct trauma to the back within the previous month * Functionally impairing back pain: A baseline score of \> 5 on the Roland-Morris Disability Questionnaire Exclusion Criteria: * Not available for follow-up * Pregnant * Chronic pain syndrome defined as use of any analgesic medication on a daily or near-daily basis * Allergic to or intolerant of investigational medications