This research study will help determine whether a medication called duloxetine can improve back pain. It is well documented that many participants who come to the ER with acute low back pain still have low back pain 3 months later. The investigator team will attempt to determine whether duloxetine can help prevent this.

Participants will be eligible to take duloxetine/placebo if insufficient relief of pain after using naproxen for 48 hours has been reported.

Inclusion Criteria:

• Present to Emergency Department (ED) primary for management of LBP, defined as pain originating between the lower border of the scapulae and the upper gluteal folds. Flank pain, that is pain originating from tissues lateral to the paraspinal muscles, will not be included.
• Musculoskeletal etiology of low back. Patients with non-musculoskeletal etiologies such as urinary tract infection, ovarian cysts, or influenza like illness will be excluded. The primary clinical diagnosis, at the conclusion of the ED visit, must be a diagnosis consistent with non-traumatic, non-radicular, musculoskeletal LBP.
• Patient is to be discharged home. Patients admitted to the hospital are more likely to be treated with parenteral medication and therefore are not appropriate for this study.
• Age 18-64 Enrollment will be limited to adults younger than 65 years because of the increased risk of adverse medication effects in older patients.
• Non-radicular pain. Patients will be excluded if the pain radiates below the gluteal folds in a radicular pattern.
• Pain duration <2 weeks (336 hours).
• Prior to the acute attack of LBP, back pain cannot occur more frequently than once per week.
• Functionally impairing back pain: A baseline score of > 5 on the Roland-Morris Disability Questionnaire

Exclusion Criteria:

• Not available for follow-up

• Pregnant or breast-feeding

• Chronic pain syndrome defined as moderate or severe pain anywhere in their body on >= 50% of days for at least three months

• Allergic to or intolerant of investigational medications

• Contra-indications to non-steroidal anti-inflammatory drugs:

  1. history of hypersensitivity to NSAIDs or aspirin
  2. active or history of peptic ulcer disease, chronic dyspepsia, or active or history of gastrointestinal bleed
  3. Severe heart failure (NYHA 2 or worse)
  4. uncontrolled blood pressure (>160/100)
  5. Glomerular Filtration Rate (GFR) <60ml/min
  6. Current use of anti-coagulants
  7. cirrhosis or acute hepatitis

• Contra-indication to duloxetine:

  1. alcohol use disorder
  2. chronic liver disease
  3. chronic kidney disease
  4. glaucoma
  5. Active use of medication for depression
  6. Score > 10 on the Patient Health Questionnaire (PHQ-9) screening instrument or thoughts of suicide, symptoms of feeling down, depressed, or hopeless

PHQ9 score >4. Now we would like to exclude patients with a PHQ9 score >10 or symptoms of feeling down, depressed, or hopeless