I got one for you.......don't join the army!
Now, on a serious note, if you have to smoke pot as pain relief or what not, the army probably wont recruit you simply because you have some kind of underlying physical debilatation. If that is the case, and you are not considered to be physically able then they probably wont want you.
I know that asthmatics and diabetics can't join......then again I don't know why you smoke weed medically so, it kinda comes down to that I guess.