How long before the first fentanyl patch starts to work?

It's peak onset. May take 24 hours with first patch, and will recapitulate the build up if a gap of 2 hours without patch occurs. Break though narcotics can be used unitl peak action.
Depends. In my practice, patients notice the patch starts to work in a day or two. They most likely have been on oral medications such as morphine for a while before the patch is started. Since i start with a low dose for my patients to acclimate to the new treatment, and not over dose them, it may take a few weeks to reach the optimal patch dose.