It's psychic (?) powers, not drugs. The psychics can get into other people's heads, read their mind, make them see things that aren't real. The little kid in the beginning had those powers but couldn't control them, and as a result was overwhelmed and drowning in his mother's mind. Also that same kid is now grown up and working for the suit guy - you can recognize him by the scar on his forehead. Besides the kid, there're two other young psychics (the 'gay' lodgers) working for the man.