Why Is Onion Bad for Dogs?

Accidental ingestion of many common household foods can poison your pets. In the case of dogs, poisons include chocolate, alcohol, avocado, grapes, coffee and onion. Onions are particularly dangerous because symptoms may not become evident until many hours after your dog eats the onion. To keep your pets safe, dispose of onions in an out-of-the-way place.

  1. Toxic Compounds

    • Onions contain allyl-propyl disulfide and n-propyl disulfide, compounds that cause abnormal reactions in dogs. These compounds are found in all forms of onions: fresh, dehydrated and cooked. They are also found in similar vegetables including chives, leeks and garlic. If your dog spends time outside, watch out for wild onions because they are also toxic.

    Symptoms

    • Outwardly apparent symptoms of onion toxicity include vomiting, diarrhea, refusal to eat and weakness. Some of the more dangerous effects of onion toxicity are visible only with a blood test. When a dog consumes onions, the toxic compounds cause changes to their red blood cells. The hemoglobin in the red blood cells oxidizes and clumps together. These clumps, known as Heinz bodies, are visible under a microscope. The toxins also produce eccentrocytes, or red blood cells that have areas with no apparent hemoglobin. Both of these conditions lead to anemia and can be fatal.

    How Much Is Bad?

    • In small amounts, onion will not harm your dog. In fact, many dog treats contain small amounts of onion powder. In general, onions become toxic when a dog consumes .5 percent of its body weight. For example, a 50-pound dog would need to eat 1/4 pound of onions. This amount, however, is highly variable. Some dog breeds -- including Shibas and Akitas -- are more sensitive to the toxic compounds in onions, and some individual dogs may be more sensitive than others.

    Treatment

    • If your dog has symptoms of onion toxicity, you must take it to the veterinarian. Some dogs will need oxygen or blood transfusions for the anemia.