A short interview in SOD with Drake Hogestyn (John) says he is doing this to Steve, in order to protect Marlena. He was told do it or else she will be harmed.
I don't think Steve is in on it, but I also think perhaps John is diluting the substance.
Actually, it would be difficult for John to doctor any of Steve's food, drink, etc. as they really do not hang together, so guess these "drinking bouts" are the only way for John currently. John would not harm Steve, and obviously whoever is ordering this doesn't want him dead.....as they would just kill him then. No, they want Steve out of commission for a while.......