Thiosulfate - Why Garlic is BAD for Dogs

Most people have never heard of Thiosulfate. This is a common chemical compound composed of two sulfur atoms and three oxygen atoms. It is found - and particularly potent - in allium vegetables like onions and garlic.

Thiosulfate is toxic to dogs. Specifically, it can cause damage to red blood cells, potentially leading to hemolytic anemia, a condition where red blood cells are destroyed faster than the body can replace them, causing symptoms like lethargy, pale gums, and rapid breathing. It can also negatively impact the ability for oxygen to move efficiently through the body. In addition to anemia, thiosulfate can also cause gastrointestinal upset, such as vomiting and diarrhea.

In cats the reaction can be far more severe, as their systems have an even more profound negative reaction to thiosulfate than canines.

Some dogs may be more sensitive to thiosulfate than others, making it difficult to determine a "safe" amount. Inversely, some dogs may be far more tolerant and while small amounts might not be harmful to those individuals, it's best to avoid feeding dogs any amount of garlic and onions (or anything else from the allium family of plants) due to the phenomenon sometimes referred to as Emergent chemical sensitization, where subjects become intolerant to some degree of a chemical or substance that was previously tolerated with little to no adverse impacts. It simply isn't worth the risk.

Garlic, onions, green onions, chives, leeks, shallots, and some types of salts (commonly used as preservatives) should all be avoided. If your dog ingests any amount of these items seek veterinary advice right away. Monitor your dog, watching for signs of anemia, gastrointestinal upset, or other symptoms that are outside of their normal behavior. Do not attempt to make your dog vomit unless instructed by your veterinarian. Many veterinarians will want to do blood work to evaluate how the body is handling the thiosulfate and determine what treatments may be most suitable. Treatments may involve both subcutaneous fluids and take home medications. Occasionally, hospitalization may be needed.

Garlic as an anti-parasitic

Louise Buckley from the Royal School of Veterinary Studies at the University of Edinburgh put it best:

"In the absence of research to demonstrate high levels of adulticidal action against a range of intestinal wormers at therapeutic, non-toxic levels in cats and dogs, clients should be advised that garlic has not been demonstrated to be an effective anthelmintic (either for multiple or single species use) for use in dogs and cats either to prevent or to treat an intestinal worm burden."

Resources:

1. Is There Any Evidence to Support the Use of Garlic as a Wormer [...] PDF

2. Why Garlic Isn't Safe for Dogs - AKC

3. Pet Poison Helpline: Garlic Is Toxic To Dogs