Apples are safe for dogs, but apple seeds are not. Apple seeds, persimmon, peach, and plum pits, as well as other fruit seeds or pits have "cyanogenic glycosides". For example, if an apple seed skin is broken as a dog eats an apple, then cyanide could be released.
Cyanide is highly toxic for dogs and can be found in a variety of fruits and toxic plants, such as cherries, apples, pears, almonds, elderberries and clover.The most common form of cyanide poisoning in dogs is from the ingestion of pits and seeds within fruit.
Are crab apples poisonous to dogs? The seeds of the apple contain the toxin cyanide. While a couple of seeds won't likely harm your dog, if eaten in large quantities, it can cause serious, life-threatening problems.

The apple seeds within the core contain cyanide, but the small amount is typically insufficient to poison a dog unless they consume a considerable amount.
Though the fruit is safe for human consumption, the leaves, stems, and seeds of the apple tree contain cyanide, which is very toxic to dogs. Symptoms of cyanide poisoning in your pet may include shock, panting, dilated pupils, and very red mucous membranes.
As we can see from the illustration, Cyanide Poisoning In Dogs From Apple Seeds has many fascinating aspects to explore.
If your dog eats some apple seeds, they should be fine. Its true that apple seeds contain cyanide, which is poisonous to dogs and people alike. However, your dog would have to eat an awful lot of apple seeds to cause cyanide poisoning.
While apples themselves are generally safe for dogs to consume, it is crucial to remove the seeds before sharing this fruit with your furry friend. Apple seed toxicity can lead to gastrointestinal distress, respiratory issues, seizures, and even cyanide poisoning in severe cases.
The Chemistry of Cyanide Poisoning and Why it Kills (thoughtco.com).