Elixirs are the strongest healing items in Monster World that are able to fully restore the player back to full health. In most games in the series, they are used automatically upon defeat. However, in Monster World IV, in order to make good use of the new Pepelogoo partner, Asha is unable to be revived automatically unless he is with her. It must be used manually beforehand.
Note: In Monster World IV, these are called "Healing Potions", but they are essentially the same thing regardless.
Healing Potion from "Monster World IV"
Elixir from "Wonder Boy in Monster World"