Onions contain compounds called disulfides and thiosulphates which can be toxic cats and dogs if ingested. The ingestion of onions causes conditions called hemolytic anemia, Heinz body anemia, and methemoglobinemia which are all manifestation of damage to red blood cells.

What happens if a dog eats onion?

Are onions toxic to dogs? Onions contain a toxic principle known as N-propyl disulfide. This compound causes a breakdown of red blood cells, leading to anemia in dogs. The toxin causes oxidative damage to your dog's red blood cells by attaching to the oxygen molecules in your dog's red blood cells.

Will a little piece of onion hurt my dog?

Generally speaking, toxicity occurs when a dog ingests more than 0.5% of their body weight in onions at one time. To put it simply, even a small amount of onion, garlic, or other toxic allium food can poison a dog easily.