Fifty-six patients entered into an open-label, randomized study to compare the efficacy and tolerability of diflunisal and naproxen in the treatment of mild to moderate pain associated with acute low back strain. Thirty-three patients completed the two-week study. No patients withdrew because of side effects, and both drugs were well tolerated. Results showed that diflunisal was more effective than naproxen (81% versus 41%) in relieving pain. Of the 16 patients taking diflunisal, 13 rated its efficacy as very good or excellent; six (35%) of 17 patients taking naproxen rated their drug similarly. Overall, diflunisal rated slightly better in efficacy and tolerability and in improving limitation of function and motion. In addition, diflunisal has a longer duration of action and thus requires less frequent dosing than naproxen.