Why would you even put it in your mouth. I have a great idea, use them as directed. Find a hairless place on your body, adhere, sit back and wait. Fentanyl is incredibly potent and has a pretty long half life as far as overall retention which makes it incredibly dangerous. You are talking a substance that is measured in micrograms as far as dosage and those patched, a 75 mic for example have about 1.5 mgs. of fentanyl in them. It is not something you play with, but to each their own.
At least you are only using Durogesic, and not Duragesic. Duros have it impregnanted in the matrix and are not so easy to overutilise. Had it bssn Duragesic I would really question you. Either way, it is not an intelligent thing to do.