Reverting back to minimalism and simplicity here:
I don't know anything about the side effects or chemical reactions that take place when Codeine is consumed, however, I do know that from my personal experiences, and from the basic observations made around me, that Marijuana doesn't appear to chaotically ruin chemical workings, and it doesn't appear to interfere with many of the intended effects that the person has wanted. If it were me, i'd do it, because it seems to be a low-risk situation.

Everything's a risk, and risks shouldn't stop you from doing anything, just make sure you're playing your cards right. And be careful.