INTRODUCTION: Nonsteroidal antiinflammatory drugs (NSAIDs) are commonly used for pain disorders such as low back pain and exist in multiple formulations; however, no systemically acting transdermal formulations are available for low back pain. Transdermal formulations can be safely administered even to patients with trouble swallowing or at risk of aspiration, and without regard to the effect of food on drug absorption. Unlike locally acting formulations, systemically acting transdermal formulations need not be applied at the target site, so dosing is simple and the burden is not on one area of the skin. A patch with the systemically acting NSAID diclofenac sodium is approved in Japan for treatment of cancer-related pain, and we hypothesized that it would be useful for controlling low back pain. METHODS: This randomized, double-blind, placebo-controlled study aimed to evaluate the efficacy and safety of diclofenac sodium patch in Japanese patients with low back pain. Eligible patients were randomized to receive diclofenac sodium patch 75 mg or 150 mg or placebo once daily for 2 weeks. The primary endpoint was pain intensity assessed on a visual analog scale (VAS). RESULTS: Primary analysis of the primary endpoint showed that both doses of the diclofenac sodium patch (150 mg and 75 mg) were superior to placebo in terms of absolute change from baseline in mean 3-day VAS score after 2 weeks' treatment; the mean difference between the active and placebo treatments in this variable was -5.67 [95% confidence interval (CI) -9.34 to -2.00] mm in the 150 mg group and -5.68 (95% CI -9.34 to -2.01) mm in the 75 mg group. Most adverse events were mild. No serious adverse events occurred. CONCLUSION: In Japanese patients, diclofenac sodium patch is effective for the relief of low back pain and is well tolerated. TRIAL REGISTRATION: JPRN number, JPRN-JapicCTI-205134.