Poisoned apple is a food item in the Blood and Wine expansion.
It can be found on the body of one of the dwarves, the Wicked Witch, purchased from the Little Flint Girl, or somewhere else in the Land of a Thousand Fables. It can be consumed as any other food but it also adds 15 toxicity points.
It's potentially useful for Alchemy build players who want the benefits of toxicity but don't want to drink potions.