Yes, yes... putting something up your nose is not an ideal situation. But there are directors and actors who will do anything for the sake of their art and forget that when you get down to it, we are just playing an expensive game of "make believe".
I did a show where one actor was supposed to watch another actor pull out some coke, cut it, and snort it. This theatre has had to do this before and they use this vitamin powder... which of course I can't remember the name of... which is harmless to injest (they had some supporting research about this). The problem, as others have suggested, is that your nasal passages aren't necessarily designed to accomodate powder and can become irritated by this action. That's something you just aren't gonna get away from. Another thing I've heard of is that instead of sniffing the powder up, the actor actually blows out hard enough to blow the powder away, making it appear that he/she sniffed it.
I'll ask the prop dept. what the name of that powder is tomorrow and get back to you.
Hope this helps!