Introduction: The rapid-onset opioid fentanyl is used to treat breakthrough pain in the management of cancer pain. Case Report: A 33-year-old woman started to receive palliative radiation for multiple spinal metastases and pathological fracture of the sacrum following surgery for right breast cancer. Although oxycodone sustained-release tablets and oxycodone powder had been used for the treatment of pain, there had been no alleviation in response to oxycodone powder at times that she experienced breakthrough pain. Therefore, the patient was switched to rapidly acting fentanyl citrate sublingual tablets. An analgesic effect was obtained, but numbness of the lower lip and taste disturbance emerged. Furthermore, pain in the lower jaw emerged and eating became difficult. Therefore, the patient was switched to fentanyl citrate buccal tablets. However, once the agents were dissolved and spread inside the mouth, the same symptoms emerged. Conclusion: Because the same symptoms were observed even after switching agents, fentanyl citrate or the common ingredient contained in both agents appeared to be the cause of this event.