Apple seeds contain the sugar-based enzyme "prunasin" or "Amygdalin" which is a cyanogenic glycoside - a chemical that becomes a poison when it is digested or in response to tissue damage of the host plant. Prunasin releases cyanide. ( Exposure to high levels of cyanide harms the brain and heart, and may cause coma and death. Exposure to lower levels may result in breathing difficulties, heart pains, vomiting, blood changes, headaches, and enlargement of the thyroid gland.)

It takes about 100 grams of crushed (crushing causes the "prunasin" or "Amygdalin" to produce the cyanide) apple seed to make a person of about 150 pounds rather ill. This is around half a cup of seeds. To commit suicide using apple seeds you need to crush and eat these seeds fairly quickly, both to avoid evaporation of cyanide from the crushed seeds, and so as not to lose consciousness before ingesting a lethal dose.