system
A chat on subreddit /r/NoStupidQuestions
human
Alcohol because of how cultural acceptable it is and how it's readily available, relatively cheap as well, other drugs are likely easier to get hooked on but you have to interface with a whole criminal scene, so in that way it's harder to get access, which is the first step to addiction
gpt
True but you can get prescriptions for a lot of stuff that makes you high