Mechanistic Evaluation of Polypharmacy Effects for Pain

Inclusion Criteria: * 18-65 years old * Lifetime history of opioid use * Medically cleared to take study medications * Chronic Lower Back Pain * Agree to use effective form of contraception throughout enrollment Exclusion Criteria: * Pregnant or breastfeeding * Have evidence of physical dependence on or current use of any central nervous system depressants including but not limited to alcohol, benzodiazepines, opioids, gabapentinoids, or barbiturates * Past 30-day use of certain pain medications * Evidence of past 7-day nonmedical drug use * Known hypersensitvity, allergy to, or history of adverse reactions to study medications * Are currently taking a medication or have a condition that is contraindicated with either study medication * Have evidence of current or past significant medical/psychiatric illness * Have a history of clinically significant cardiac arrhythmias or vasospastic disease * Have evidence of renal or hepatic impairment * Current suicidal ideation or behavior, or a history of a suicide attempt within the past 2 years * Have any condition that may increases the risk for respiratory depression