Why are onions bad for dogs?

When a dog digests onion, compounds called organosulfoxides are converted into substances that attack red blood cells. The cells become fragile and break apart, and the body cannot replace them quickly enough. The result is hemolytic anemia, where the blood carries less oxygen than it should.

No amount of onion is known to be safe. The risk depends on the dog's size, the form of onion and how much was eaten, and a small amount of concentrated onion powder can matter for a small dog. That is why vets want to hear about any exposure rather than have owners guess.

Cooking does not make onion safe. Onion powder, dried onion flakes, fried onions and onion soup mix are more concentrated than fresh onion, so a small amount goes a long way. Gravy, stock cubes, sausages, stuffing, pizza, curry and baby food can all contain onion.

Some breeds, including Akitas and Shiba Inus, are thought to be more sensitive to onion toxicity.

Vomiting and diarrhea may appear within a day. The more serious signs linked to anemia often come 2 to 5 days later: tiredness, weakness, pale gums, fast breathing, a racing heart, reluctance to walk, and red or brown urine. Because of this delay, a dog that looks fine the next morning still needs advice.

Damage can build up when a dog is given small amounts of onion regularly, such as table scraps from a family that cooks with onion every day. That is why vets advise against sharing any food containing onion, even in modest portions.