The primary objective of this study is to evaluate the safety and tolerability of MK-0663B (etoricoxib/tizanidine) among participants with moderate-to-severe acute low back pain.

Inclusion Criteria:

• Have acute low back pain;
• Onset of acute low back pain must be <6 weeks prior to screening;
• Have acute low back pain corresponding to a rating of ≥ 4 on a 0-10 point Low Back Pain Questionnaire and ≥ 7 on the Roland Morris Questionnaire;
• For women of childbearing potential, have a negative serum pregnancy test at Visit 1 (Screening Visit) and agree to remain abstinent, use barrier, or non-hormonal implanted contraceptives from study start until 14 days after the last dose of study drug;
• Be willing to limit alcohol use, if any, to 2 drinks or equivalent per day for the duration of the study and follow-up period;
• Be willing to avoid unaccustomed physical activity (e.g., starting a new weight lifting routine) for the duration of the study and follow-up period;
• Be willing to avoid chiropractic care, ultrasound and physical therapy from screening to Day 9;
• Be willing to avoid cold applications and other alternate treatments (e.g. massage, acupuncture etc.) on Day 1 and within 30 minutes prior to scheduled pain assessments on Day 3 and Day 8.

Exclusion Criteria:

• Has low back pain that is related to, or known to be caused by malignancy, inflammatory disease (rheumatoid arthritis, ankylosing spondylitis, psoriatic arthritis, Reiter's syndrome), osteoporosis, ochronosis, vertebral fracture, infection, juvenile scoliosis, congenital malformation, or fibromyalgia. Note: the presence of radiographic degenerative disc disease is not an exclusion;
• Has signs or symptoms consistent with a neurologic, infectious, or fracture-related cause of acute low back pain;
• Has chronic low back pain and is experiencing an acute flare-up of the chronic back pain condition;
• Has radicular or myelopathic pain;
• Has a history of lumbar spine surgery;
• Is involved in an active civil lawsuit or workman's compensation claim pertaining to his/her low back pain;
• Has symptomatic depression that could interfere with the completion of the questionnaires;
• Is mentally or legally incapacitated, has significant emotional problems at the time of the study, or has a history of major psychiatric disorder, including any history of psychosis;
• Has a Body Mass Index (BMI) ≥ 40;
• Is allergic to, or has a history of a significant clinical or laboratory adverse experience associated with tizanidine, etoricoxib, methocarbamol, meloxicam, or any non-steroidal anti-inflammatory drug (NSAID);
• Is allergic to acetaminophen/paracetamol;
• Is currently a user (including "recreational use") of any illicit drugs, or has a history (within 5 years) of drug or alcohol abuse;
• Has participated in another investigational drug study within the last 4 weeks;
• Has uncontrolled hypertension;
• Has systolic blood pressure (SBP) < 105 or diastolic blood pressure (DBP) < 65;
• Has a history of orthostatic hypotension;
• Has ischemic heart disease, peripheral arterial disease and/or cerebrovascular disease;
• Has a positive human immunodeficiency virus (HIV) test result, hepatitis B surface (HBS) antigen, or hepatitis C virus (HCV) test result;
• Has a history of hepatitis/hepatic disease that has been active within the previous year;
• Has a history of gastric, biliary, or small intestinal surgery or disease that results in clinical malabsorption;
• Has a history of neoplastic disease;
• Has any personal or family history of an inherited or acquired bleeding disorder;
• Is expected to undergo a planned surgical procedure or invasive diagnostic procedure during the course of the study;
• Is pregnant or breast-feeding, or expecting to conceive within the projected duration of the study;
• Has an active peptic ulcer or a history of inflammatory bowel disease.