My response:
Once he turns 18, he's an adult and responsible for his own problems - - not you.
However, if for example, he uses your car and causes an accident, you would also be Statutorily "on the hook" for monetary damages because it was your car.
Also, remember, even though he gets into trouble with drugs, and it's on his head for doing so, and it's his responsibility, don't you think it would "impact your life" when police authorities come busting into your home at 2:00 a.m. ?
IAAL