The antipoison elixir is dropped by hellhounds during the Vengeance! saga. Drinking it will remove the effect of poison for two minutes. Eventually, its effect will wear off and you will once again become poisoned. It does not leave behind an empty vial when consumed, but previously, if emptied it does. After an update, attempting to empty it only causes the message "Nothing interesting happens."
It does not provide immunity against poison. If you are poisoned by a Dungeon spider while it is in effect, the poison would be refreshed.