Mechanistic Study of Duloxetine in Breast Cancer Patients With Chronic Pain

Inclusion Criteria: 1. Female patients at least 25 years of age 2. Diagnosis of stage 0-III breast cancer within 12 years prior to enrollment. All indicated surgery, chemotherapy, and/or radiation therapy must have been completed at least 12 weeks prior to enrollment. Concomitant endocrine therapy and targeted therapies such as palbociclib, pertuzumab, and trastuzumab are permitted. 3. Pain that developed or worsened since breast cancer diagnosis and is not due to identifiable traumatic event or fracture 4. Patient-reported worst pain score between 5 and 10 (inclusive) on a 0-10 scale (assessed verbally) 5. Female patients must be at least 1 year postmenopausal or surgically sterile; or must agree to use a medically acceptable form of contraception 6. Willing to withdraw from selective serotonin reuptake inhibitors (SSRI) and tricyclic antidepressants (TCA) prior to treatment initiation 7. Patients who are currently taking non-steroidal anti-inflammatory drugs (NSAIDs) (e.g., ibuprofen, naproxen, meloxicam, gabapentin, pregabalin) and/or opioid pain medications must remain on a stable dosage throughout the duration of the study 8. Able to provide informed consent and willing to sign an approved consent form that conforms to federal and institutional guidelines. Exclusion Criteria: 1. Prior use of duloxetine or milnacipran. 2. Prior or current use of venlafaxine specifically for treatment of pain (prior or current use for treatment of other indications, such as hot flashes, is permitted, although cases currently taking venlafaxine must discontinue use prior to study treatment initiation) 3. Patients must not be taking any contraindicated medications listed on the duloxetine package insert including the following: phenothiazines, propafenone, flecainide, linezolid, or anticoagulation medication (e.g., heparin, warfarin, or direct oral anticoagulants); treatment with monoamine oxidase inhibitor within 14 days prior to registration. 4. Thumbnail abnormalities on either hand (such as due to chemotherapy or trauma, or artificial nails) that are likely to alter pain perception during testing 5. Peripheral sensory neuropathy at the thumbs bilaterally that interferes with function and/or activities of daily living 6. Significant risk of suicide based on the Investigator's judgment 7. History or behavior that would, in the Investigator's judgment, prohibit compliance for the duration of the study. 8. History of alcohol or other substance abuse or dependence within t…