This is a Phase III, longitudinal, multicenter, randomized, double-blind clinical trial. The aim of the study is to evaluate the efficacy and safety of a fixed-dose combination of etoricoxib and tizanidine compared to etoricoxib alone in patients with acute low back pain associated with muscle spasms.

Researchers will compare the efficacy of a fixed-dose combination of etoricoxib and tizanidine versus etoricoxib alone in the treatment of acute low back pain associated with muscle spasms. Efficacy will be assessed by evaluating the average change in pain among patients who report improvement over the 7-day follow-up period. Adverse events related to the interventions will be recorded throughout the study.

Participants will:

• Be randomized into one of two intervention groups (Group A or Group B).
• Attend three in-person clinic visits: Day 0 (baseline), Day 3, and Day 7 of follow-up.
• Receive follow-up phone calls on Days 1 and 5.
• Be allowed to take 500 mg of acetaminophen as rescue medication if needed, with prior authorization from the Principal Investigator.

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