Onions are very toxic to cats. Potentially fatally toxic, in fact. They cause abnormalities in the red blood cells leading to their destruction and severe anemia (called Heinz body anemia due to the structure created within the blood cells), and can also cause kidney failure. This can happen relatively quickly, and with only a small amount of onion. It has been known to happen after cats ate baby food with onion powder as an ingredient. Garlic is in the same plant family, and can cause the same toxicity. Cats have died from people giving them garlic tablets as a way to prevent fleas (which garlic does NOT do, and has been studied scientifically). These plants are also potentially toxic to some dogs, but the risk is nowhere nearly as serious as with cats.
So keep all of this in mind. NEVER give your cats anything with any onions or garlic in them. This is a very serious toxicity, and requires intense hospitalization and even blood transfusions once symptoms begin.- Taken from A Vet's Guide to Life