What, now I am an authority?
I am only able to picture a person leading a class seeing obvious behaviour...all the signs of drug use are pretty blatant in most cases. The users who don't show it as much are of course less of a outright danger on the face of it, but are still being irresponsible to their partners and (hopefully) friends in the dojo.
If a person has a physical issue ...you mentioned ADD then the teacher should be told up front. They will see it pretty soon anyway if it is bad enough to warrent concern.
Once again, a grey area....
My first thought is that if you can't see the problem, it isn't enough of a problem to matter. But that belies the issues of responsibility that each student on the mat should address for themselves FIRST.