Ha, my actors that I had for that scene actually snort all kinds of shit: pills, coke, and God knows what else. They wanted to do the real thing, but I really never liked the idea of having that shit on set.
Shit, I’d use the real thing as long as you’ve got access to it. Unless your filming
in a public place or something. Just tell everyone who you don’t trust that it’s flour
or any one of the things people have suggested in this thread. At least for the
shots where they’re gonna be snorting it. You can use anything to represent large
amounts still in the bag.