3. OnionsWhat makes onions poisonous to cats? It is an ingredient called thiosulphate which affects their blood. It can actually make their red blood cells burst and is very dangerous for both cats and dogs. You wouldn’t notice the symptoms for a few days but even a small amount of raw or cooked onions are extremely toxic for our furry friends. When you’re cooking and be careful when you have guests over that do not have pets, make sure they know that some human foods are poisonous to your cat.