What do you do when your boyfriend gets mad at you because you won't buy him Robitussin?

So he can drink the whole bottle and get high. The first time I bought him the Robitussin, I thought there was a legitimate reason. That is until I awoke one late night and heard him screaming in the living room that I had a call on hold..at like 3:30am. He was high. He has a past with hard drugs but I was under the impression that he was finished with it. His mother peddles Xanax pills to him like skittles (and I hate her) and he smokes marijuana and bitches at me if I don't give him money. He breaks my personal belongings and doesn't even care. It seems like all he wants these days are drugs. He can't even spend a regular evening with me anymore without bitching that he needs something. I'm really tired of it. I think he's dragging me down, in just about every way possible. My energy, my money, everything.