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Apples contain cyanide, too much of which can kill you.

Unripe peaches can kill you as well, such as they did to King John in 1216.

Cherries are another potentially deadly fruit.

If the seed inside a cherry is damaged in any way,

even if you were to bite into it, releases prussic acid (hydrogen cyanide), 

which can result in death. 

so if yo wanna kill urself in a very slow process just eat those fruits.