Visiting Vet: Onions and garlic are toxic for cats and dogs

It doesn’t matter if plants are natural.


Every time my friend Gail brings her pup for a veterinary visit, she also brings me a little bag. Usually when clients do this, it’s a bag of poop. Not Gail. Her bag contains lovely homegrown garlic. I used to grow garlic myself, but I’m just too busy to garden. Besides, I don’t exactly have a green thumb. Another friend once gave me a “stick plant.” “It already looks dead,” she joked, “so it won’t matter if you kill it.” But I digress.

Garlic. Last week someone left a handout about garlic on my porch. Delivered anonymously, the page, titled “Common Questions About Garlic,” appeared to be from a catalog for “Natural Supplements for Horses, Dogs, and People.” Multiple passages, highlighted in orange, touted the use of garlic as dewormer, heartworm preventive, flea and tick repellent, and so on. Sure, back in the day, some of us old hippie veterinarians tried these. Nowadays I simply say “No, thank you.” Here’s why.

Allium is the genus of vegetation that includes garlic, onions, shallots, leeks, and ramps. These pungent, delicious plants have been used both for cooking and medicinal purposes for thousands of years. As far back as 1550 B.C.E., garlic was used to treat typhus, dysentery, cholera, and influenza. In contemporary times, studies suggest garlic may be a “promising candidate” for preventing and treating a variety of human health conditions. It has some anticancer, cardioprotective, anti-inflammatory, and antimicrobial effects, as well as helping lower blood pressure and blood sugar. There is even limited clinical evidence it may be helpful in neurodegenerative disorders such as Alzheiner’s, Parkinson’s, Huntington’s, and Lou Gehrig’s disease. That said, if garlic were such a miraculous panacea, no one in Italy would ever get sick.

Why do I reject this orange-highlighted handout encouraging me to give garlic to my patients? Because of the Pet Poison Hotline’s 2023 list of Top 10 Toxins. For cats, onions are No. 3, garlic No. 4. For dogs, onions and garlic are No. 8. Why can we eat garlic with impunity (other than garlic breath), but our pets can not? Because dogs and cats are not little people. The red blood cells of each species contain species-specific hemoglobin with a species-specific number of sulfhydryl groups that stabilize the blood cells’ structure. Garlic and onion have sulfur-containing oxidants that bind to these sulfhydryl groups, damaging red blood cells. People only have two sulfhydryl groups on our hemoglobin — not enough for onions and garlic to cause significant damage. Dogs have four. Cats have a whopping eight.

Every time a cat or dog eats garlic or onions, some red blood cells undergo oxidative injury. These cells rupture and die. This is called hemolysis. It’s not a big deal if it’s just a few cells, but if a lot of red blood cells die, your pet now has hemolytic anemia, a potentially fatal condition. Garlic or onion ingestion can cause gastrointestinal signs (vomiting, diarrhea, belly pain, and loss of appetite) that may appear right away. The more serious hematological issues can take three to five days, or even longer, to appear. Symptoms include pale gums, increased heart and respiratory rate, weakness, lethargy, and collapse.

There is a classic toxicology adage: The dose makes the poison. This is particularly true for onions and garlic. I know a person who freaked out when her big dog ate two onion bagels. “Is he going to die?” she cried. No. If two onion bagels could kill a Labrador, there wouldn’t be a single Jewish dog living today. So how many onion bagels can Bialy the beagle eat safely? How much garlic can Pesto the Persian cat tolerate? It’s complicated, and often difficult to calculate. Dried, powdered, flaked, and granulated products are more concentrated, and hence more toxic than fresh produce. Garlic is more toxic than onions. Hemolytic anemia can occur after one large ingestion, but also from repeated small ingestions over a longer period of time. “Deodorized” products (such as several “supplements” on the orange-highlighted handout) have had some of the disulfides removed. This makes them less toxic (and less smelly) … but since those disulfides are likely the medically active components, such products probably have few if any beneficial properties.

What about things like homemade dog biscuits, containing onion and garlic powder? How much powder? How much does Bialy weigh? How many biscuits did he eat? Notice I didn’t ask, “How many biscuits did you give him?” Because dogs will be dogs. What if Bialy steals the whole bag of biscuits, and eats them all? If one biscuit isn’t toxic, what about 20? Dogs have no restraint. Dogs will eat anything. I read about one who ate 10 packets of dehydrated onion soup mix, and developed severe anemia. Another consumed five raw onions. Dogs. Luckily, cats tend to be more discriminating, but ingestion of very small amounts can be potentially toxic, especially onion or garlic powder, and especially if fed repeatedly.

The reality is that pets rarely consume enough onions or garlic to be truly dangerous, but when they do, it is very serious. If you know Pesto or Bialy has consumed more than a very small amount, it’s worth checking with an animal poison hotline to do the math for you, and determine if intervention is warranted. If the amount ingested is potentially toxic, it is prudent to try to induce vomiting within two hours.This is pretty easy with dogs, harder with cats. Cats only vomit when they feel like it. On your bed. During your dinner party. They often won’t barf when you really need them to. Fortunately, cats are less likely to binge-eat than dogs. On the other hand, once signs appear, there is no specific antidote, only supportive care. Fluids, oxygen, blood transfusions. Then just hope the pet survives.

So thanks, Gail, for the garlic, which I will happily use for my stir-fry dinner. But “no, thanks” to the anonymous handout’s “natural” garlic supplements.There are safer and more effective options for Bialy and Pesto.