IntroductionManagement of patients with cancer suffering from neuropathic pain refractory to opioids and gabapentinoids remains an important challenge. Duloxetine is one of the choices after first-line treatment fails. The efficacy of duloxetine has been reported in patients with non-cancer disease and in chemotherapy-induced peripheral neuropathy, but no randomised clinical trials have examined its effects on neuropathic cancer pain refractory to first-line treatment. The objective of this study is to assess the analgesic efficacy of duloxetine in patients suffering from neuropathic cancer pain refractory to opioids and gabapentinoids.Methods and analysisA multi-institutional, prospective, randomised, double-blind, placebo-controlled, two-parallel trial is planned. The inclusion criteria are adult patients with cancer suffering from neuropathic cancer pain refractory to opioids and gabapentinoids, patients with a Numerical Rating Scale (NRS) pain score of 4 or higher and patients with a total Hospital Anxiety and Depression Scale score of less than 20. Patients with chemotherapy-induced peripheral neuropathy are excluded. The study will take place at 14 sites across Japan. Participants will be randomised (1:1 allocation ratio) to a duloxetine intervention group or a placebo control group. Evaluations will be made at baseline (T0 randomisation), day 0 (T1), day 3 (T2) and day 10 (T3). The primary endpoint is defined as the difference in NRS score for pain intensity (average over the previous 24 hours) at T3 between the duloxetine and placebo groups. A sample size of 70 patients will be examined between July 2015 and March 2018.Ethics and disseminationEthics approval was obtained at all participating sites.The results of this study will be submitted for publication in international peer-reviewed journals and the key findings presented at international scientific conferences.Trial registration numberUMIN000017647; Pre-results.Protocol version2.2, 26 April 2017.