Objective: To assess the time of onset and difference in analgesic efficacy of oral dexketoprofen compared with oral diclofenac in patients with acute lower limb injury. Design: A prospective, double blind, randomised controlled trial. Interventions: Patients who fitted the study criteria were given either 25 mg oral dexketoprofen trometamol or 50 mg sodium diclofenac immediately after triage; baseline and 15 minute pain scores were then recorded for one hour. Results: 122 patients were studied (diclofenac = 57 and dexketoprofen = 65). There were no significant differences in age, sex, type of injury, or baseline pain scores between the two groups. The differences in group mean pain scores between diclofenac and dexketoprofen at 15, 30, 45, and 60 minutes were; 0.53 (95% confidence intervals 20.03 to 1.09), 0.70 (0.16 to 1.24), 0.89 (0.32 to 1.47), and 0.83 (0.21 to 1.45). Odds ratios for a decrease in pain score of at least 1 from baseline (on the 11 point scale) when given dexketoprofen rather than diclofenac at 15, 30, 45, and 60 minutes were; 2.66 (1.19 to 5.98), 3.52 (1.60 to 7.73), 4.48 (1.72 to 11.65), and 5.54 (1.90 to 16.15). Corresponding odds ratios for a decrease in pain score of >2 were; 6.88 (1.48 to 32.0), 3.79 (1.59 to 9.01), 5.19 (2.29 to 11.78), and 5.87 (2.68 to 12.88). Conclusions: Dexketoprofen trometamol is an effective and rapidly acting analgesic for the treatment of acute musculoskeletal injuries. D exketoprofen trometamol is the water soluble salt of the S-isomer of the racemic non-steroidal anti-inflammatory (NSAID) drug ketoprofen.1 S-isomers rotate polarised light to the right, while R isomers rotate it to the left. Dexketoprofen acts by inhibition of cyclooxygenase, thus diminishing prostaglandin synthesis. It has been shown that the stereo-isomer of ketoprofen is about 3000 times more potent than the R-isomer at doing this. 2 3 Additionally in vitro studies have shown greater COX-1 inhibition with this Sisomer of ketoprofen, compared with other racemic NSAIDs. 4 The trometamol salt form of dexketoprofen was formulated to improve the pharmacokinetics of the orally administered drug. The maximum concentration is greater and the time to it less, with the trometamol salt compared with the free acid form of dexketoprofen. This study was designed to assess whether this new form of NSAID using enantiomer selectivity would perform more effectively in the ED setting than a standard NSAID. The criteria for effectiveness were time of onset of effect and size of effect. Diclofenac was chosen as the comparator as it is the most widely used NSAID in England.