I was utterly disappointed when both of my boys moved out at 18, dropped out of college and smoked pot. This really hurt their father, who had been wonderful to them.
One of them has moved back in and is in his second year in college. He is getting a degree in applied computing. He also got on a health kick, works out, and doesn't smoke anymore that we know of.
My older son lives in a condo with a roommate. He works full time, supports himself, and is in his third year of college.
The changes are like a miracle. I will say however, the boys lost a lot of trust from their dad.