Antitoxin does clear poison...kind of. So I used it to clear poison in my game and it went away, but I noticed that Poison can actually have multiple stacks to increase damage. What I am guessing is when you use it you had more than one stack, so I am inferring that Poison cleanse can only clear one stack of poison...that said, I hope Restore can clear the whole stack, because that would make Restore staves significantly weaker.