There certainly was a lot of talk about other substances on this thread before I cleaned it up. I was beginning to think this topic was the worst invention ever.

I'll have to say guns, although I know humanity well enough to know we'd invent--and have invented--equally awful ways to kill each other, even without firearms. Guns certainly have played a significant role in the history of our world, now that I think about it.

Intolerance is a good choice. I'd also have to put pantyhose on the list.