As you watch someone carelessly munching on an apple core, some vague alarm bells go off in your head. Didn't you hear somewhere that apple seeds are poisonous? Well, apple seeds can indeed be poisonous, but it takes quite a few of them to kill you and only if they have been crushed. Apple seeds (and the seeds of related plants, such as pears and cherries) contain amygdalin, a cyanogenic glycoside composed of cyanide and sugar. When metabolized in the digestive system, this chemical degrades into highly poisonous hydrogen cyanide (HCN). A lethal dose of HCN can kill within minutes.
Thankfully, there are several factors that make death-by-apple-seed very unlikely. First, the amygdalin is accessible only if the seeds have been crushed or chewed; a whole unbroken seed will pass right through. Second, the human body can process HCN in small doses, so a couple of chewed seeds are usually completely harmless. Finally, the average adult would need to eat anywhere from 150 to several thousand crushed seeds (depending on the apple variety) to be at risk of cyanide poisoning. The average apple contains only about five to eight seeds. So unless someone is eating their 18th consecutive apple core and has been meticulously chewing all the seeds, they should be fine with their occasional absentminded core chomping.