A Cochrane review included 31 studies with 1 237 participants.
Seventeen studies (392 participants with neuropathic pain, average 22 participants per study) provided efficacy data for acute exposure to opioids over less than 24 hours. Sixteen reported pain outcomes, with contradictory results; 8/16 reported less pain with opioids than placebo, 2/16 reported that some but not all participants benefited, 5/16 reported no difference, and 1/16 reported equivocal results. Six studies with about 170 participants indicated that mean pain scores with opioid were about 15/100 points less than placebo.
Fourteen studies (845 participants, average 60 participants per study) were of intermediate duration lasting 12 weeks or less; most studies lasted less than six weeks. All demonstrated opioid efficacy for spontaneous neuropathic pain. Meta-analysis demonstrated at least 33% pain relief in 57% of participants receiving an opioid versus 34% of those receiving placebo. The overall point estimate of risk difference was 0.25 (95% CI 0.13 to 0.37), NNTB of 4.0 (95% CI 2.7 to 7.7). When the number of participants achieving at least 50% pain relief was analyzed, the overall point estimate of risk difference between opioids (47%) and placebo (30%) was 0.17 (95% CI 0.02 to 0.33), NNTB 5.9 (3.0 to 50.0). Constipation was the most common adverse event (34% opioid versus 9% placebo) followed by drowsiness (29% opioid versus 14% placebo), nausea (27% opioid versus 9% placebo), dizziness (22% opioid versus 8% placebo), and vomiting (12% opioid versus 4% placebo). More participants withdrew from opioid treatment due to adverse events (13%) than from placebo (4%). Conversely, more participants receiving placebo withdrew due to lack of efficacy (12%) versus (2%) receiving opioids.
, classified as short-term (less than 24 hours; 14 studies, 267 patients) or intermediate-term (median = 28 days; range = eight to 70 days; 9 studies, 460 patients). Four drugs were tested in the latter group: morphine in 4, oxycodone in 3, methadone in 1 and levorphanol in 1 trial.The short-term trials had contradictory results, but all intermediate-term trials demonstrated opioid efficacy for spontaneous neuropathic pain. Pain intensity was 13 points lower (visual analog scale 0 to 100) after opioids as compared to placebo (95% confidence interval –16 to –9; meta-analysis of seven intermediate-term studies). The most common adverse events were nausea (33% opioid versus 9% control: number needed to treat to harm [NNH] 4.2) and constipation (33% opioid versus 10% control: NNH 4.2). Drowsiness (NNH 6.2), dizziness (NNH 7.1), and vomiting (NNH 8.3) were also common.Comment: The level of evidence is downgraded by study quality and imprecise results.