Are Opioids Effective in Relieving Neuropathic Pain?

Abstract Neuropathic pain (NP) and its treatment are considered to constitute an unmet need, with a high-multidimensional impact on society and the sufferer. The broad spectrum of opioid analgesics is considered beneficial for acute pain, yet these drugs pose serious controversial issues due to the potential for adverse behavior and a higher chance of tolerance and addiction in long-term use. Opioids like other first-line medications for NP, will not be useful for every patient suffering from chronic NP. However, due to their possible adverse effects, opioids are considered as second- or third-line medications by various guidelines. Therefore, this literature review was conducted to evaluate the status of opioids in NP and to asses if any recent research has shed further evidence on their efficacy or the contrary. The literature reviewed showed that the mechanisms underlying NP, may themselves contribute to the reduced effect of opioids in this condition. Also, various genetic polymorphisms affecting pharmacokinetic and pharmacodynamic factors are discussed, providing further evidence for the variability in opioid response. Although opioids may reduce NP, nociceptive pain tends to be more responsive to opioids compared to NP. Also, opioids seem to be more effective in intermediate term studies of up to 12 weeks and being mostly effective in peripheral NP compared to  supraspinal NP and being least effective in central NP. However, there is still no robust evidence that any specific opioid agent is better than any other one for NP, but it is possible that opioids targeting multiple mechanisms may provide benefit. A limitation of many trials is the lack of consideration for the comorbid psychological aspects of NP, which tend to lower opioid analgesia.