Don't remember. I think I missed a lot of the last season though. I do know Beavis got jacked up on Caffiene pills during the movie.
When the homeless man is teaching them to panhandle. he says "You won't make a dime if you act like that." Beavis says "Whoa, you think somebody might give us a dime?!" When they go to sell blood in a very early episode, the nurse asks "are you two on any medication?" and they both just laugh. Also, there is one episode where they are watching videos and there is a quick shot of them on the couch in which I'm pretty sure one of them has a joint. So yes it was implied but only through subtle references.