yeah, this. It doesn't matter if he's doing it for a good reason or not, if it doesn't come into his interactions with students.

Like, maybe he's not buying pot to help with migraines, maybe he's buying coke to get high on weekends. Maybe he's buying meth or heroin, and somehow still manages to be a good teacher. So what? The advice for the student should be "watch yourself with the teacher, be wary of invitations to his house, and be sensible -- as with all adults you don't know well personally."