STUDY DESIGN: A systematic review of randomized and/or double-blinded controlled trials. SUMMARY OF BACKGROUND DATA: The use of muscle relaxants in the management of nonspecific low back pain is controversial. It is not clear if they are effective, and concerns have been raised about the potential adverse effects involved. OBJECTIVES: The aim of this review was to determine if muscle relaxants are effective in the treatment of nonspecific low back pain. METHODS: A computer-assisted search of the Cochrane Library (Issue 2, 2002), MEDLINE (1966 up to October 2001), and EMBASE (1988 up to October 2001) was carried out. These databases were searched using the algorithm recommended by the Cochrane Back Review Group. References cited in the identified articles and other relevant literature were screened. Randomized and/or double-blinded controlled trials, involving patients diagnosed with nonspecific low back pain, treated with muscle relaxants as monotherapy or in combination with other therapeutic methods, were included for review. Two reviewers independently carried out the methodologic quality assessment and data extraction of the trials. The analysis comprised not only a quantitative analysis (statistical pooling) but also a qualitative analysis ("best evidence synthesis"). This involved the appraisal of the strength of evidence for various conclusions using a rating system based on the quality and outcomes of the studies included. Evidence was classified as "strong," "moderate," "limited," "conflicting," or "no" evidence. RESULTS: Thirty trials met the inclusion criteria. Twenty-three trials (77%) were of high quality; 24 trials (80%) were on acute low back pain. Four trials studied benzodiazepines, 11 nonbenzodiazepines, and 2 antispasticity muscle relaxants in comparison with placebo. Results showed that there is strong evidence that any of these muscle relaxants are more effective than placebo for patients with acute low back pain on short-term pain relief. The pooled relative risk for nonbenzodiazepines ...