It seems to be fairly well known that onions are toxic to dogs. I have witnessed the panic in veterinary clients’ eyes when they come rushing into the clinic because their dog ate some human food that contained a little diced onions. While onions and garlic are indeed toxic to dogs and cats, it brings up several issues worth clarifying.
First and foremost, toxic does NOT mean deadly! As a veterinarian who specialized in toxicology, I am always happy when the mere word “toxic” raises alarm in pet parents. This is not because I want them to think that what I do is so important. Rather, it is because there are some really awful toxins and poisons out there that can really harm our pets, our citizens, and our environment. But let me say clearly now that not all toxins are instantly deadly!
It was Paracelsus who in the 15th century is quoted as saying, “all substances are poisons; there is none which is not. It is the dose that determines when.” Basically, he was saying that everything is potentially toxic. And that is quite true. But it does not mean that everything toxic is necessarily deadly. When it comes to onions and garlic it is the dose and the length of exposure that matter most in pets.
Onions and garlic contain a large number of sulfur-containing compounds that can be harmful to dogs and cats. These compounds are found throughout the edible and non-edible parts of onions and garlic, meaning that they are found in the peels, stems, and leaves as well as the bulb parts that are used in human foods. One of these compounds is responsible for the tearing effect people get when they chop onions. After dogs and cats eat onions these compounds are absorbed into the blood. Once in the bloodstream, they bind to hemoglobin molecules within red blood cells causing the hemoglobin to precipitate onto the cell membrane. The red blood cell membranes become damaged with what are called Heinz bodies, and can be identified through a microscope when the blood is smeared onto a microscope slide. If a dog or a cat were to eat a small amount of either onions or garlic some of their red blood cells would be damaged. In most dogs and cats this does not usually cause any disease or concern if it only happens once and the amount eaten was low. But when large amounts are eaten all at once or over time enough red blood cells could be damaged and result in a functional anemia. This means simply that the red blood cells are no longer able to transport oxygen efficiently to the organs and tissues that are depending on the oxygen carried by the blood. This then results in all sorts of issues for the animal. While most animal species are sensitive to these effects, most severe cases occur in livestock that might graze through fields of growing onions and eat massive amounts. Cats are significantly more sensitive to the effects of onions and garlic because their hemoglobin molecules are more sensitive to the sulfur compounds.
So now the bigger question to answer is, how much onions or garlic does it take to cause illness in a dog or cat? That is difficult to answer because even though fresh and dried onions and garlic are potentially toxic, the disulfides are volatile and the content can vary over time. The research has shown that dogs have developed severe anemia after eating about a tablespoon of onions per kg of bodyweight daily (about 4.5 tablespoons per day for a 10-lb dog ). For cats about a teaspoon per kg of bodyweight has the same effect (about 4 teaspoons per day for a 10-bl cat.) It is best to prevent any exposure in pets but with the understanding that daily exposures are potentially the most harmful.
The key is prevention. Don’t let your dog or cat eat any human foods containing onions! Because even the bits that we do not use in cooking also are harmful be certain to empty your trashcans after cooking to prevent your curious dog or cat from rummaging through and finding the disposed scraps. If you think that your dog or cat has eaten more than the dangerous amounts mentioned above, take them immediately to receive proper veterinary care.
This is Dr. John Tegzes, the toxvet, wishing you a poison-free day!
John H. Tegzes, MA, VMD, Dipl. ABVT
Dr. John Tegzes is a veterinarian who specializes and is board-certified in Toxicology. He graduated from the School of Veterinary Medicine at the University of Pennsylvania, and completed residency-training in diagnostic and clinical Toxicology in the Vet School at the University of California, Davis. He has worked as a veterinarian in companion animal practices in Portland, OR, and in Toxicology at the Oregon Poison Center and the California Poison Control System. Currently he is a Professor of Toxicology in the College of Veterinary Medicine at Western University of Health Sciences in southern California. He resides in Los Angeles, California in the happy company of two dogs, two cats, and a sun conure.