I've been mildly addicted for a while, so I kinda have an idea what I'm talking about here, but of course I'm probably wrong.
Codeine can be injected quite safely, if you know what you're doing. Keeping a clean needle and such.
I assume you're talking about using AC&C's or some other codeine + something product. When I use only codeine (not too often), I usually do a cold water extraction like one would for vicodins.
I've also never injected it. I have a friend who has, and he reccomends strongly against it, almost had a leg amputated. But, that was because of his idiocy (durrrrrty needles, injected straight saline once by accident). Make sure that you only use the solution and that theres no leftover powder sitting at the bottom. AND ONLY DO IT IM!!! Thighs and asses work well for this, so i'm told, anyway.