^ Right, and for codeine to become metabolized into morphine, it needs to pass through the liver, which means you have to swallow it. In the end, the effect will come on slowly, and as others have mentioned, there's a ceiling to it.
As for heroin, the different RoA's allow the drug to enter your bloodstream much faster, creating a sort of rush that you would never experience with codeine.
It does help for wd's, though, but to what extent depends entirely on how big your H habit was in the first place, and how efficient your liver is at metabolizing codeine. Some people just don't feel it at all, while others can practically nod off higher doses.