Aim: to estimate efficacy, tolerability, and safety of Ambenium® parenteral [phenylbutazone sodium 400 mg and lidocaine hydrochloride 4 mg] used for treatment of patients with low back pain. Methods. A total of 100 patients with low back pain (LBP), who received a single intramuscular injection of Ambenium® parenteral, were included in the study. The follow-up period was 3 days. The change in pain severity on VAS (mm) was selected as a primary outcome measure for efficacy. The pain syndrome severity was estimated before administration of the drug (visit 1) and 4, 6, 8 and 12 h after the drug administration, as well as during visits 2 (day 2) and 3 (day 4). The McGill Pain Questionnaire, muscle syndrome index (MSI), Roland-Morris Disability Questionnaire pain estimates were used as the secondary diagnostic criteria. The percentage of patients with the positive response relative to baseline (the decrease in pain severity on VAS by more than 50%) was also determined. Results. The dynamics of the primary efficacy measure (pain severity on VAS) was as follows: 7.4 ± 0.92 cm (mean ± standard deviation) during visit 1, 5.6 ± 1.49 cm after 2 h, 4.6 ± 1.74 cm after 4 h, 3.0 ± 1.79 cm after 8 h; the VAS score was 2.3 ± 1.70 cm after 12 h (р<0.001). The share of patients who responded to therapy (with the decrease in pain severity on VAS by more than 50% relative to baseline) was 84% [95% CI: 75.0 – 91.0] 24 h after the first administration, 94% during visit 3 [95% CI: 87.0 – 98.0], р<0.001. The secondary efficacy criteria (muscle syndrome index and McGill Pain Questionnaire score) showed significant positive dynamics (р<0.001). No systemic adverse events were reported. Conclusion. The study showed that a single intramuscular injection of Ambenium® parenteral was an efficient, well-tolerated and safe treatment option for low back pain. Administration of a single drug dose makes it possible to ensure a significant clinical effect reflected in the pain severity decrease and restoration of the patient’s motor activity.