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In Japan, transdermal fentanyl (Durotep Patch) was launched in March 2002, and it was regarded as making opioid rotation possible. When changing from morphine to transdermal fentanyl, the efficacy ratio of 1:150 is used in Japan as well as in many other countries. However, the ratio of 1:100 is used in Germany. As a result, a dose increase in transdermal fentanyl is often required to control pain. We studied transdermal fentanyl use in the Aichi Cancer Center (ACC) to investigate the actual conversion ratio and appropriate switching by following up 144 patients (81 men, 63 women) who had received transdermal fentanyl in the ACC from March 19, 2002, to April 30, 2003. Transdermal fentanyl improved pain control in patients who had difficulty in tolerating oral medication or in continuing morphine because of side effects. Regression analysis indicated that the efficacy ratio of oral morphine to transdermal fentanyl was 1:78. As the fentanyl dosage was excessive even in some patients who followed the recommended morphine/fentanyl conversion of 150:1, it is dangerous to use the conversion ratio of 78:1 at first. Morphine side effects were reduced in some patients who changed to transdermal fentanyl, but there was no reduction in those who needed high-dose morphine for rescue analgesia. Therefore it is safe and effective to use low-dose transdermal fentanyl in the beginning and to control pain promptly using rescue morphine based on the present recommended dosage. For opioid rotation, quick-acting opioids other than morphine are expected to be launched in Japan.