One hundred and thirty-four patients with moderate-severe, acute low back pain were randomized to receive a single intramuscular injection of diclofenac-thiocolchicoside (n=68) or diclofenac only (n=66). The primary outcome of interest was the mean change in Visual Analog Scale (VAS) pain score from baseline to 3 hours. Secondary outcomes of interest include the percentage of patients showing a >30% reduction in pain intensity, muscle spasm improvement (assessed via finger-to-floor distance), and the incidence of adverse events. VAS pain scores were significantly lower in the combination group vs. the diclofenac-only group at 1 and 3 hours post-treatment. Moreover, finger-to-floor distance and the proportion of patients with a >30% improvement in VAS pain scores were significantly better in the combination group. No adverse events were reported in the combination group, while 2 cases of dizziness were reported in the diclofenac-only group. This suggests that combining diclofenac with thiocolchicoside can safely and effectively reduce pain for patients with acute, moderate-severe low back pain, more so than diclofenac only.