Investigation of Residual Amount of Fentanyl in Used Transdermal Fentanyl Patches: Effects of Person Applying Patch and Application Site

Though a change from oral morphine dosage forms to transdermal fentanyl patches achieved good pain relief in 70% of cancer patients, the remaining 30 % still showed poor pain control or fentanyl vitiation. In some cases when pain control was poor, effective pain-relief was only sustained for 3 days. This suggests that the absorption of fentanyl was being hampered in such cases.In view of these findings, we investigated the relationship between the amount of fentanyl released and plasma fentanyl concentrations. To estimate amounts of fentanyl delivered into the skin, we measured the amount of fentanyl remaining in fentanyl patches after 3 days of continuous use. Amounts of fentanyl remaining in the patches ranged from 41-58%, which was close to the design residual amount of 40%. However, the amount of fentanyl release varied between patches applied to patients at home and those applied to hospitalized patients, and the fentanyl was more steadily released from patches applied to the sides of the body or back than those applied to the chest. The fentanyl release also varied with the caregivers who were applying them.Thus, it is essential to apply patches according to the instructions to ensure steady release. In this study, we observed that plasma fentanyl concentrations rapidly decreased for some patients 3 days after patch application so in such cases, patches should be changed every 2 days.