just ask them one question: "have YOU, yourself verified the crap you were told? cuz u hear a lot of crap from all kinds of sources, the burden of verifying credibility is on u"

and then say this "just a few years ago, marijuana was said to fund terrorismbut to most people it was quickly obvious that it's a plant that can be grown anywhere and by anyone, and most of good quality stuff is grown domestically, at best, funding an old hippe and at worst funding a drug dealer. no of which constitutes to a terrorist. aint no terrorist is gonna sell weed for money, they sell opium and heroin, wholesale, 10000 miles away in afghanistan, and they sell cocaine 5000 miles away, wholesale, in columbia! they need the money, and when u compare prices, u will understand that selling weed is considered small business compared to other hard drugs"

u can in fact just say this and quit talking, either they get it or they dont. cuz in either case, at that moment, they probably ought to shut up, most likely cuz they have not verified the crap.

this has worked with me, in both my govt classes in college, shutting up buncha idiots at once

then, even in case of a negative feedback, u can use your stockpile of the 17000+ scientific researches in favor of marijuana, one at a time. most likely the most arrogant people will just quit the discussion, but u win part of the observing crowd, anyone of them who's got some brains of their own left!!!!