Cats should not eat onions. The reason is that onions are a pungent food; feeding them to cats can cause significant damage to their red blood cells, leading to poisoning symptoms such as vomiting blood and bloody urine. Therefore, cat owners are strongly advised against feeding their cats onions or foods containing onions. If a cat accidentally eats onions and experiences adverse reactions, induce vomiting immediately and take the cat to the vet for examination and treatment. 
Why cats can't eat onions:
1. Onions contain a chemical called thiosulfate, which is toxic to cats. If a cat eats onions, the thiosulfate will be absorbed into its bloodstream, damaging red blood cells and causing anemia.
2. Consuming large amounts of onions can, in severe cases, lead to the need for blood transfusions. Even small amounts of onions can cause vomiting and diarrhea.
3. Toxins in onions can damage red blood cells in cats. Onions concentrate these toxins, increasing their concentration and making them more dangerous for cats. Symptoms usually appear a few days after the food is digested, and severe cases may require a visit to the vet.
4. Onions are highly irritating, and cats may experience respiratory and digestive tract irritation and adverse reactions after consuming them.
5. Onions can cause hemolytic liver failure in cats. If a cat accidentally ingests an onion, it should be taken to the vet for examination and treatment immediately.
