Onions are toxic to dogs.
The toxicity is dose dependent, so the bigger the animal, the more onion need be consumed to cause a toxicity. Onion toxicity causes a Heinz body anemia. Heinz bodies are small bubble-like projections which protrude from a red blood cell and can be seen when the cells are stained. If numerous red cells are affected and rupture, anemia can result. It is a form of hemolytic anemia.