Efficacy and Safety of Etoricoxib/Tizanidine Versus Etoricoxib for Acute Low Back Pain Associated to Muscle Spasm

Inclusion Criteria: * Willing to participate in the study and provide written informed consent. * Diagnosis of acute low back pain, either as a first-time episode or following a previous episode within the past 6 months, with a duration not exceeding 6 weeks. * Pain associated with lumbar muscle spasm, assessed through physical examination (muscle hypertonia and/or reflex scoliosis) and clinical history. * Patient reports moderate to severe pain intensity (VAS ≥ 40 mm). * Patients with a Neuropathic Pain Questionnaire (DN4) score \< 4. * For women of childbearing age who are sexually active, the use of an acceptable contraceptive method (barrier and/or hormonal) as determined by the investigator. * At the discretion of the Principal Investigator (PI) or treating physician, the patient is eligible for treatment with the investigational product and may benefit from it. Exclusion Criteria: * Patients with a history of hypersensitivity to any components of the investigational product (Etoricoxib/Tizanidine) or their derivatives (as reported in the medical history and clinical interview). * Patients participating in another clinical trial involving an investigational treatment or who have participated in one within the past 2 weeks before the study begins. * Patients whose participation in the study may be influenced (e.g., employment relationship with the research center or sponsor, vulnerable populations, etc.). * Patients for whom the investigational drug is contraindicated for medical reasons. * Positive pregnancy test, pregnant women, breastfeeding women, or those planning a pregnancy during the study period. * Significant history of gastrointestinal disorders (e.g., active gastric ulcer, Crohn's disease, ulcerative colitis, etc.). * Prior treatment with opioids and/or NSAIDs, including COX-2 inhibitors, as reported in the medical history within 48 hours before study enrollment. * Patients with a history of congestive heart failure: NYHA class II-IV. * Concomitant use of potent CYP1A2 inhibitors (e.g., fluvoxamine, ciprofloxacin, etc.). * History of alcohol or drug abuse within the past year. * Patients with a history of established ischemic heart disease, peripheral arterial disease, and/or cerebrovascular disease (including patients who have recently undergone coronary revascularization procedures or angioplasty). * Patients with a history of seizure disorders, status epilepticus, or grand mal seizures. * History of liver disease classified as Child-Pugh A, B, or C, as reported in the medical history or clinical interview. * History of acute or severe renal failure (glomerular filtration rate \<30 ml/min/1.73 m²), as reported in the medical history or clinical interview. * Patients with a history of chronic musculoskeletal pain (e.g., fibromyalgia, Paget's disease, cancer-induced metastatic bone pain). * Patients diagnosed with low back pain due to a history of major trauma within the past 12 months (e.g., vertebral fracture, post-traumatic spondylolisthesis), a visceral disorder (e.g., dysmenorrhea, history of endometriosis), or a neuropathic component. * At the physician's discretion, any disease that affects prognosis and prevents outpatient management, which must be evaluated by the principal investigator to determine the subject's eligibility. * History or presence of any disease or condition that, in the investigator's opinion, could pose a risk to the patient or confound the efficacy and safety assessment of the investigational product. * Patients with symptoms suggesting an active COVID-19 infection (e.g., fever, cough, dyspnea) and/or contact with a suspected or confirmed COVID-19 case in the last 14 days. * Oncology patients (except for basal cell skin cancer) or those with severe illnesses who, in the investigator's opinion, have a severe prognosis or a life expectancy of less than one year