Dogs should never eat cooked onions. Onions, whether raw, cooked, powdered, or in any form, contain a toxic compound called N-propyl disulfide that damages their red blood cells, leading to hemolytic anemia. This condition reduces the ability of the blood to carry oxygen, which can cause severe health issues including organ failure and sometimes death. Cooking does not reduce the toxicity of onions for dogs, so all parts and forms of onions should be avoided in a dog's diet. Even small amounts can be harmful, especially for small dogs or those with preexisting health conditions. If a dog consumes onions, veterinary attention should be sought promptly, particularly if symptoms like weakness, pale gums, rapid breathing, or reddish urine appear.