You could say something along the lines of: "Hey, Friend, what's so good about weed? Why are you always smoking it?" If he's smoking it for medical purposes, then just drop it, but if it's illegal and he's doing it to "look cool", then I would further ask questions about it.
Our immune systems have improved over the years due to improvements in the medical fields. I also believe your brain has a bigger effect on how sick you actually allow yourself to become
