PR: prolonged release; CR: controlled release; ER: extended release; MOR: μ-opioid receptor; NRI: noradrenaline reuptake inhibition; IR: immediate release; OXN: oxycodone; TAP: tapentadol; RCTs: randomized controlled trials; OA: osteoarthritis; LBP: low back pain; OPO: other opioids; NP: neuropathic pain. Here, OPRM1 is linked to neuropathic pain.