Fentanyl remaining in a transdermal system following three days of continuous use

To determine whether there was a sufficient amount of fentanyl remaining in a patch that had been used continuously for 3 days to warrant establishment of disposal policies to prevent diversion of fentanyl.

Nine patches were applied and removed by hospice nurses after 3 days of continuous use on hospice patients with cancer. Patches were analyzed for the remaining fentanyl contents using the Coat-A-Count Fentanyl radioimmunoassay. Five 2.5-mg patches and four 10.0-mg patches were opened, solubilized with methanol, diluted with water, and analyzed in duplicate. An unused 2.5-mg patch also was analyzed as a control and showed a 94% recovery of fentanyl. A methanol blank was negative for fentanyl.

The study determined the amount of fentanyl in milligrams remaining in a used patch. Using pharmacokinetics principles, this quantity was compared with a potential lethal dose of fentanyl.

Analysis showed 0.7-1.22 mg remaining in the 2.5-mg patches and 4.46-8.44 mg remaining in the 10.0-mg patches. These numbers represent 28-84.4% of the original contents. Using the pharmacokinetic values of the volume of distribution of 4L/kg and a potential lethal blood concentration of 3.7 micrograms/L, one can calculate the potential lethal dose for a 70-kg person to be 1036 micrograms. This is well within the amount remaining in the patch. This study also demonstrated that a wide patient variability exists in the absorption of fentanyl from the patch.

There is a sufficient amount of fentanyl available for abuse and misuse after 3 days of therapeutic use. Adequate disposal policies currently are not established and need to be implemented.