Oral transmucosal fentanyl has been indicated for the management of breakthrough pain in patients with cancer. Fentanyl sublingual tablets(FST)have been approved for use in Japan since 2013. However, the optimal use of FSTs has not been well-elucidated. In this case, a 73-year-old man with rectal cancer and third lumbar vertebral metastasis was treated with 100 μg FST and 12.5 μg/h fentanyl patch every day for the management of cancer-related breakthrough pain. After receiving the fourth dose of FST, the patient was unconscious for 2 days. However, his respiration was stable. This case shows that due care should be taken while administering FSTs to patients, specifically geriatric patients with bone metastasis and hypoalbuminemia.