I just googled this, and found that there has been experimentation of giving fentanyl as a patch to cats, I assume they know exactly the
dose and they were clearly doing it under lab conditions. But
fentanyl can easily kill people, they think they are taking a little pill but it has a huge dose, and they die. Someone trying to guess and put together a dose for their cat at home is taking a huge risk. If a vet recommends a pain killer for a cat, he or she will be able to advise on the dosing.