The Dangers of Onion Toxicity

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The organosulfoxides (sulfurs) in onions, garlic, leeks and chives react with the cell membranes of the red blood cells of dogs, causing the cells to lyse (explode).  Cats are even more sensitive to the lytic effects of  allium (onion family) toxicosis than dogs.

It is a common myth that a small amount of onions or garlic is not harmful to dogs.  In fact, many homemade dog treat recipes include garlic powder as a flavoring because dogs tend to love it so much.  Dogs are more tolerant of garlic than onions, and small amounts of either often do not produce effects that are noticed.

foxy roxy
Foxy Roxy - recently adopted from Nebraska Humane Society. Smaller dogs are more greatly affected than larger dogs by the same amount of onions.


However, I believe that any amount of garlic or onions is unacceptable, because it always causes damage on a cellular level, whether or not we notice the effects of the damage and label it “toxic.”

common definition of veterinary toxin – taking in a substance at a high enough level to cause measurable damage or issues that are noticed by others

correct definition of toxin – poison

A small amount of garlic or onion ingestion will cause a small amount of subclinical hemolysis.  That is, a small amount will cause a small amount of red blood cell explosion.  Dogs need their red blood cells to oxygenate their brains and other important organs.

A moderate amount of garlic or onion ingestion will probably cause your pet to feel light headed and lethargic, which may go unnoticed.

A large amount of garlic or onion ingestion will cause clinical signs that are felt by the dog and noticed by the people.  Signs may take several days to develop.   Signs include:

  • abdominal pain
  • vomiting
  • diarrhea
  • lethargy
  • fast heart rate
  • fast breathing
  • anemia – low levels of red blood cells
  • paleness secondary to anemia
  • icterus – yellowing of the skin, gums or whites of the eyes, in this case secondary to hemolysis

hem/o – blood

lysis – rupture or destruction

hemolysis – rupture or destruction of red blood cells

anemia – low levels of red blood cells

Heinz bodies – microscopic changes to red blood cells seen with this particular toxicity as well as some others.  Heinz bodies are little swellings seen on the cell membranes of red blood cells before they lyse.  They are signs of a weakened cell membrane.

Onion and garlic toxicity are rarely fatal, but they can cause serious illness secondary to the red blood cell destruction and resulting anemia that they cause.  The anemia caused by onion toxicity is a regenerative anemia, meaning the body will be triggered to make new red blood cells.  This process can take up to a week to begin, and until then, cell damage continues to occur.

If your pet has ingested onions, garlic or foods containing either, call your veterinarian right away.  At early stages, the toxicity can often be treated with supportive care and detoxification.  At later stages, oxygen therapy and blood transfusions are sometimes required along with supportive care and monitoring of anemia.

Please, even if the treat recipe says to add garlic, even if your dog loves leftover stew with onions, even if he has worked hard for the leeks he dug from the garden, never allow your dog to ingest foods from the onion family.  The resulting toxicity is rarely fatal, uncommonly dangerous, sometimes uncomfortable, but always occurs.