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.

How much onion can kill a dog?

"Consumption of as little as 15 to 30 g/kg in dogs has resulted in clinically important hematologic changes,” says Hohenhaus. “Onion toxicities are consistently noted in animals that ingest more than 0.5% of their body weight in onions at one time.” So, think that one fourth of a cup can make a 20-pound dog sick.

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.