Probably going to be judge, but this is a serious question

Why would it be a bad thing to take one of my boyfriend's muscle relaxers besides the fact that it's his prescription medication and illegal? It's Baclofen 20mg and I would obviously only take half. He never finishes the bottle so he doesn't mind if I take a couple for when my cramps are really bad and nothing is helping. It's not like I'd be abusing the drug, just taking one or two the week of my period