Nonsteroidal anti-inflammatory drugs (NSAIDs) are a main tool to treat acute nonspecific low back pain (NLBP). However, no factors that influence the efficacy of these drugs have been identified to the present day.Objective: to evaluate the therapeutic effect of and tolerance to NSAIDs (meloxicam) in treating acute NLBS and to identify the factors influencing the efficacy of this drug.Patients and methods. A study group consisted of 2078 patients (mean age 46.3±13.4 years; women 56.6%) with acute NLBS who had been treated in real clinical practice. The level of pain was estimated using a 0–10 point numerical rating scale (NRS). Initially, the pain level averaged 6.69±1.65 scores; 57.0% of patients were noted to have severe pain (≥7 NRS scores). Pain at rest persisted in 32.0% of patients; that at night was in 19.0%; sensation of stiffness in 60.7%, irradiation to the leg in 28.2%, and lumbar ischialgia in 9.6%. 70.2% of patients had been previously treated with NSAIDs for NLBS, while only 28.0% rated their efficacy as good. All the patients were prescribed meloxicam at a dose of 15 mg/day for a period of up to 2 weeks. 86.1% of the patients received meloxicam intramuscular for 2 days, then orally; 13.9% took the drug only orally. 52.3% of the patients also used muscle relaxants; 17.4% received oral or intramuscular B vitamins. The study estimated the rate of complete pain relief when NSAIDs were used for up to 2 weeks.Results and discussion. Complete pain relief was achieved in 75.2% of patients. 83.7% of patients rated the effect of treatment as good or excellent. Undesired drug reactions were recorded in 4.6% of patients. Female gender had no effect on treatment outcome (odds ratio (OR)=0.967; 95% confidence interval (CI), 0.795–1.177; p=0.763). Age over 65 years, the first NLBS episode, and a good NSAID effect in a history were associated with the best treatment result: OR=2.053 (95% CI, 1.5920–2.642), p<0.001; 1.415 (1.09–1.836), p=0.009; and 1.937 (1.513–2.481), p<0.001, respectively. Severe pain (≥7 NRS scores), persistent pain at rest and at night, and especially lumbar ischialgia indicated the worst result: OR=0.481 (95% CI, 0.393–0.588), p<0.001; 0.559 (0.441–0.709), p<0.001; 0.511 (0.413–0.631), p<0.001; and 0.346 (0.256–0.466), p<0.001, respectively. NSAIDs in combination with muscle relaxants and B vitamins versus NSAID monotherapy did not increase the likelihood of pain relief: OR=0.827 (95% CI, 0.594–0.889), p=0.02 and 0.917 (0.804–1.1201), p=0.452, respectively.Conclusion. Meloxicam at a dose of 15 mg/day is an effective and safe drug to treat acute NLBS. Patient gender has no effect on treatment outcome. Age over 65 years, the first NLBS episode, and a good response to NSAIDs in a history are associated with the best treatment results; and severe pain, persistent pain at rest and at night, irradiation to the leg, and lumbar ischialgia are related to the worst result. NSAIDs in combination with muscle relaxants and B vitamins did not improve treatment outcomes.