Abstract: Only one study met our inclusion criteria. Participants were men and women (mean age 67 years), with postherpetic neuralgia, complex regional pain syndrome, or chronic postoperative pain. They were experiencing inadequate relief from non-opioid analgesics, and had not previously taken opioids for their neuropathic pain. The study used an enriched enrolment randomised withdrawal design. It was adequately blinded, but we judged it at unclear risk of bias for other criteria. Transdermal fentanyl (one-day fentanyl patch) was titrated over 10 to 29 days to establish the maximum tolerated and effective dose (12.5 to 50 µg/h). Participants who achieved a prespecified good level of pain relief with a stable dose of fentanyl, without excessive use of rescue medication or intolerable adverse events ('responders'), were randomised to continue with fentanyl or switch to placebo for 12 weeks, under double-blind conditions. Our prespecified primary outcomes were not appropriate for this study design, but the measures reported do give an indication of the efficacy of fentanyl in this condition. In the titration phase, 1 in 3 participants withdrew because of adverse events or inadequate pain relief, and almost 90% experienced adverse events. Of 258 participants who underwent open-label titration, 163 were 'responders' and entered the randomised withdrawal phase. The number of participants completing the study (and therefore continuing on treatment) without an increase of pain by more than 15/100 was 47/84 (56%) with fentanyl and 28/79 (35%) with placebo. Because only 63% responded sufficiently to enter the randomised withdrawal phase, this implies that only a maximum of 35% of participants entering the study would have had useful pain relief and tolerability with transdermal fentanyl, compared with 22% with placebo. Almost 60% of participants taking fentanyl were 'satisfied' and 'very satisfied' with their treatment at the end of the study, compared with about 40% with placebo. This outcome approximates to our primary outcome of moderate benefit using the Patient Global Impression of Change scale, but the group was enriched for responders and the method of analysis was not clear. The most common adverse events were constipation, nausea, somnolence, and dizziness. There was no information about other types of neuropathic pain, other routes of administration, or comparisons with other treatments. We downgraded the quality of the evidence to very low because there was only one study, with few participants and events, and there was no information about how data from people who withdrew were analysed. There is insufficient evidence to support or refute the suggestion that fentanyl works in any neuropathic pain condition.

Plain-language summary: In January 2016 we searched for clinical trials where fentanyl was used to treat neuropathic pain in adults. We found one small study that did this and met our requirements for the review. The study had a complicated design. Study participants first received fentanyl (as one-day skin patches) for one month. Those who responded to therapy (achieved a predetermined level of pain relief) were then randomly allocated to continue receiving fentanyl or placebo for 12 weeks. The participants had one of three different types of neuropathic pain and had not taken opioids before. There were only 163 people in the 12-week comparison with placebo. The study found that more people taking fentanyl had pain relief than those taking placebo. About 1 in 7 participants stopped taking fentanyl because of side effects, and 1 in 5 did not get a good level of pain relief in the first part of the study. Almost half of those who continued into the second part of the study also stopped. The most common side effects were constipation, nausea (feeling sick), somnolence (feeling sleepy), and dizziness. These are typical side effects with opioids such as fentanyl. There was so little information from this single study that we concluded there was no convincing evidence to support or reject a meaningful benefit for fentanyl over placebo. We rated the quality of the evidence as very low because there was only one study, with few participants and events, and an unusual design. Very-low-quality evidence means that we are very uncertain about the results.