The objective of this study was to evaluate the influence of cancer cachexia on pain control in cancer patients receiving a transdermal fentanyl patch (FP) and to investigate whether dry skin was a factor related to cancer cachexia and uncontrolled pain. We retrospectively reviewed the medical records of 77 patients receiving FP treatment for the first time, who were classified into cancer cachexia and non-cancer-cachexia groups, according to European Palliative Care Research Collaborative criteria. On day 7 after FP administration, the mean FP dose and morphine equivalent dose (MED) in the cancer cachexia group were significantly higher than in the non-cancer-cachexia group. Additionally, in the cancer cachexia group, there was a significantly larger degree of variation in pain intensity over 7 d than in the non-cachexia group. In patients who were switched from FP to morphine injection, the mean pain intensity and MED on day 3 after morphine injection were significantly lower than those immediately before morphine injection. Subsequently, to investigate whether dry skin was involved in poor pain control in the cancer cachexia group, transepidermal water loss (TEWL) was compared between 15 additional patients classified into cancer cachexia and non-cancer cachexia groups; the mean TEWL in the cancer cachexia group was found to be significantly lower. Our data suggest that cancer cachexia may be a risk factor for poor pain control in patients receiving FP treatment, and that uncontrolled pain in FP treatment may be caused by the inhibition of fentanyl transdermal absorption due to dry skin.