Apple Seeds are poisonous. see this

Technically, apple seeds are poisonous. Apple seeds, when exposed to certain chemicals (such as stomach acid) can produce cyanide. One would have to ingest a lot of apple cores to feel the effects of the cyanide. They are poisonous, but it is still safe to eat the occasional apple seed.

 if you eat enough apple seeds or cherry pits, you could die. Apple seeds contain cyanogenic acids. Cherry pits, and seeds from related fruits, including peaches, plums, almonds, pears, and apricots, contain cyanogenic glycosides. Your body can detoxify small quantities of cyanide compounds. If you accidentally eat a cherry pit in a pie or swallow an apple seed or two, you'll be fine. Actually, if you swallow several seeds whole, you would absorb a minimal amount of the toxic compounds. Chewing the seeds makes them much more hazardous to your health. Children and pets are much more likely to suffer poisoning from eating the seeds than adults.

Symptoms of mild poisoning include headache, dizziness, confusion, anxiety, and vomiting. Larger doses can lead to difficulty breathing, increased blood pressure and heart rate, and kidney failure. Reactions can include coma, convulsions, and death from respiratory arrest. There are several treatment options, but the main thing is to limit the absorption of the chemicals from the seeds. Basically, this means it's important to seek immediate medical attention if a child or pet is known to have eaten several seeds. Usually, the plan of action is to pump the stomach or induce vomiting. Antidotes are available, but they are somewhat controversial. If you or someone you know eats a seed or two, don't worry... as I said, your body is well-equipped to detoxify small quantities of cyanide compounds. They naturally occur in several foods. However, if you were wondering whether or not it's true that the seeds and pits are toxic and potentially lethal... yes, apple seeds and cherry pits are poisonous.