From personal experience, getting legit transdermal fent patches are extremely hard to get unless you're a cancer patient or know of someone who is that can get you some. It's actually easier to get straight fent (or fent analogues, for that matter) nowadays from drug dealers nowadays--or at least, from where I live at. Merely smoking the fent made me OD, and that's coming from someone who had prior experience and moderate tolerance to heroin, so specifically IV'ing isn't necessary, although it would potentiate a higher rate success for death. I used to shoot up heroin, but haven't done so in over a year; you have to make sure that you've registered the needle into a vein. One of the primary signs you have is when you pull the plunger; and if you see a plume of blood entering the syringe, then you've hit a vein. It could also be an artery (blood from an artery is bright red, whereas the blood for veins are a dark red), but you'll know whether or not if you've hit an artery due to the instant, sharp pain you get if you try to shoot it there. This was all self-taught, by the way. Made a few mistakes in the beginning, but gradually got the hang of it to the point where I shot up some meth for some random guy who didn't want to do it himself.