You can unpoison yourself with a Pot, Scroll, or by Ranger11, spell. But you cannot un-crit yourself.
Also, once you are poisoned, you can put on your Proof item, and when the save is called for, your item will kick in, and your poison is instantly gone.
One thing I do is, if I am in an area where I know I might get poisoned (spiders, scorps, trap, etc.) then I put on the Proof item. But I would NEVER ever take off my Fort item for a Proof Against Poison item!! Fort is way more important.
As Rangers we can take care of ourselves with Poison. But we NEED the Fortification far far far more.
There are ML8 Robes of Heavy Fort, but I'd use a Mod Fort belt until level 11. Generally Robes wouldn't be useful to us unless our DEX gets past the mid 20's range.BTW, if I am wrong about not being any ML 10 Heavy Fort items, I would love to know that!
Just watch your back until you do get Heavy Fort at 11. When you see that Troll or Ogre winding up for the massive bash, do a quick tumble back, then slice him into ribbons. Take advantage of your Ranger spring attack!!