Apple seeds, cherry pits, and other seeds from the Prunis family contain a natural substance called amygdalin, which can be enzymatically degraded into glucose, benzaldehyde, and hydrogen cyanide. Hydrogen cyanide is toxic. Here's the first MSDS I found on hydrogen cyanide when I ran a search, which says ingesting less than 1 mg/kg can kill you. So... that's bad news for your body, even if there are heathful compounds in the seeds, unless you have the ability to detoxify the cyanide... and you do. If the dose is low enough, you can survive exposure to cyanide just fine, with no ill effects. If the dose is too high, then you're in trouble. That's where eating too many of the seeds can kill you. I don't have an opinion on whether or not eating a small number of the seeds is healthful.
I read that some people use the terms amygdalin, Laetrile, and vitamin B17 interchangeably, but there are sources which distinguish between them. Amygdalin is the substance that occurs naturally in plants. Laetrile is a trade name for a synthesized compound, laevo-mandelonitrile-beta-glucuronoside. This compound is chemically similar to amygdalin, but isn't the same molecule. I'm not entirely clear on whether B17 is supposed to be amygdalin or Laetrile, but neither compound is recognized as a vitamin. The 'vitamin' designation seems to have been part of a marketing ploy by the Mexican clinic that sells Laetrile. I don't see much information about the natural compound, amygdalin, but there is scientific research that indicates Laetrile does not cure cancer.
- Laetrile and amygdalin are not the same molecule.
- Both compounds can be metabolized to produce cyanide in the body. Too much of either compound is toxic.
- Laetrile has not been shown to cure cancer.
- There may or may not be health benefits from eating apple seeds or related seeds, but if you eat too many of them, you can get sick and may die.