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 aprevious 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 acceptablecontraceptive method (barrier and/or hormonal) as determined by the investigator.

• At the discretion of the Principal Investigator (PI) or treating physician, thepatient is eligible for treatment with the investigational product and may benefitfrom it.

Exclusion Criteria:

• Patients with a history of hypersensitivity to any components of the investigationalproduct (Etoricoxib/Tizanidine) or their derivatives (as reported in the medicalhistory and clinical interview).

• Patients participating in another clinical trial involving an investigationaltreatment or who have participated in one within the past 2 weeks before the studybegins.

• Patients whose participation in the study may be influenced (e.g., employmentrelationship 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 apregnancy 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 reportedin 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 arterialdisease, and/or cerebrovascular disease (including patients who have recentlyundergone coronary revascularization procedures or angioplasty).

• Patients with a history of seizure disorders, status epilepticus, or grand malseizures.

• History of liver disease classified as Child-Pugh A, B, or C, as reported in themedical history or clinical interview.

• History of acute or severe renal failure (glomerular filtration rate <30 ml/min/1.73m²), as reported in the medical history or clinical interview.

• Patients with a history of chronic musculoskeletal pain (e.g., fibromyalgia, Paget'sdisease, cancer-induced metastatic bone pain).

• Patients diagnosed with low back pain due to a history of major trauma within thepast 12 months (e.g., vertebral fracture, post-traumatic spondylolisthesis), avisceral disorder (e.g., dysmenorrhea, history of endometriosis), or a neuropathiccomponent.

• At the physician's discretion, any disease that affects prognosis and preventsoutpatient management, which must be evaluated by the principal investigator todetermine 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 ofthe 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 14days.

• Oncology patients (except for basal cell skin cancer) or those with severe illnesseswho, in the investigator's opinion, have a severe prognosis or a life expectancy ofless than one year