I don't mix my weed with anything except for maybe a little vape duff if I'm running low on weed, but duff is still weed, it's just been vaporized already.

I usually leave a little tobacco in the end of a blunt so that I can throw away the roach without wasting weed, but when I'm smoking a joint/bong/bowl it's all weed. Though I think I found a piece of a crackrock in one of my nicks the other day! Freaked me the hell out...