In 2013, oral transmucosal fentanyl was first approved in Japan for reducing breakthrough pain(BTP). The development of BTP may contribute to less-effective analgesia, a reduced satisfaction with analgesia therapy, obstacles in daily life, mood disorders, and an increased use of healthcare resources. In most BTP, both the duration from BTP onset to its maximum intensity and the overall duration of BTP episodes are relatively short. The need for improved rapid pain relief for BTP in this setting has led to the development of rapid-onset opioids(ROOs), including oral transmucosal fentanyl citrate(OTFC). OTFC is characterized by a rapid onset and short duration of action. Therefore, the drug is optimally indicated for BTP in patients whose pain cannot be sufficiently controlled by the rapid-release preparation, and whose sleepiness due to the carry-over effect of analgesic action interferes with daily living. In addition, the drug can be used for patients who find it difficult to use oral preparations. Furthermore, since fentanyl is the main active ingredient, less severe side effects, such as constipation, are expected. OTFC may also be safely used for patients with renal dysfunction. Since the drug has many characteristics that differ from conventional rapid-release preparations, it is important to become familiar with the use of OTFC. In order to address improving the QOL of cancer patients, a comprehensive assessment of the patient, including the risk of BTP being inadequately controlled by conventional rescue preparations is necessary.