Phase III longitudinal, multicenter, randomized, double-blind clinical trial. The aim of this study is to evaluate the efficacy and safety of the Fixed-Dose Combination of Etoricoxib/Cyanocobalamin Versus Etoricoxib in Patients With Acute Low Back Pain

Researchers will compare the fixed-dose combination of Etoricoxib/Cyanocobalamin versus Etoricoxib in acute low back pain by comparing the proportion of patients that reported an improvement in pain during the 7 days of follow up. The adverse events related to the interventions will be registered during follow up.

Participants will:

• Be randomized into one of the 2 intervention groups (A or B)
• Visit the clinic in 3 occasions (day 0, day 3 of follow up and day 7 of follow up)
• In case needed the patient could take 50 mg of tramadol, as a rescue medication, previous authorization of de principal investigator

Inclusion Criteria:

• Agree to participate in the study and give written informed consent
• Patient with pain reported as moderate to severe intensity with ≥ 40 mm on the Visual Analog Scale (VAS).
• At least 4 points on the "Douleur Neuropathique-4 items" (DN-4) scale.
• Diagnosis of acute low back pain as a first-time episode or a previous episode 6 months before the enrollment day and lasting no more than 6 weeks.
• Women of childbearing potential under a medically acceptable method of contraception

Exclusion Criteria:

• Patients participating in another clinical trial involving an investigational treatment or participation in one within 4 weeks prior to study start
• Patients in whom the participation in the study may be influenced (employment relationship with the research site or sponsor, inmates, etc.)
• At medical discretion, a disease that affects prognosis and prevents outpatient management, for example, but not limited to: end-stage cancer, kidney, heart, respiratory or liver failure, mental illness or with scheduled surgical or hospital procedures
• History/presence of any disease or condition that, in the opinion of the Investigator, could pose a risk for the patient or confusing the efficacy and safety of the investigational product
• Patients in whom the study drug is contraindicated for medical reasons
• Patients with allergy or hypersensitivity to the active substance of the study drugs, related products or excipients (Etoricoxib of Cyanocobalamin)
• Pregnant women, women breastfeeding or planning a pregnancy during the conducting the study
• Significant history of gastrointestinal diseases (e.g., gastric ulcer, Crohn's disease, Ulcerative Colitis, etc.)
• Active opioid and/or NSAID treatment including COX-2 inhibitors, reported in the medical record within the last 72 hours of study entry.
• Patients with a history of congestive heart failure: NYHA II-IV.
• Concomitant use of strong CYP1A2 inhibitors (e.g., fluvoxamine, ciprofloxacin, etc.).
• Patients with a history of alcohol or drug abuse in the last year
• Patients with a history of ischemic heart disease, peripheral artery disease, and/or cerebral vascular disease (including patients who have recently undergone coronary revascularization or angioplasty)
• Patients with a history of seizures, epileptic status and/or grand mal seizures
• History of chronic liver failure Child-Pugh A, B, and/or C
• History of acute renal failure (glomerular filtration rate <30 ml/min/1.72 m2)
• Patient with a history of chronic pain associated with fibromyalgia, Paget's disease or bone pain induced by metastatic cancer
• Low back pain due to a history of major trauma in the past 12 months (e.g., vertebral fracture, post-traumatic spondylolisthesis) or due to a visceral disorder (e.g., dysmenorrhea, history of endometriosis).
• Patients with symptoms suggestive of COVID-19 infection (fever, cough, dyspnea) and/or contact in the last 14 days with a suspected or positive patient for COVID-19