Greet said it best when she said that boundaries are set to protect and satisfy us NOT the addict.
In my estimation and experience, I have to agree with those who said crossing boundaries have to have a consequence. They are not worth a damn if they don't have cause and effect.
Using drugs in ones home is dangerous and illegal. Overlooking it can have serious consequences of their own. YOU have decided that you do not want this in your home; therefore, you established a boundary. When that boundary is compromised, you must, for your own protection, enforce it and deliver a consequence that satisfies and protects YOU. If he cannot adhere to what you find acceptable or unacceptable, you need to let him know that until he can respect your wishes and boundaries, ______ will happen. (you fill in the blank with the consequence but it has to have some teeth or it won't mean a thing)
Remember, the addict cares little about anyone's boundaries and trying to talk logic with an addict is a waste of time. The boundaries and the subsequence consequences are for YOU.