Although garlic does have many beneficial uses, some species variations occur that warrant cautionary advice.
All members of the onion family contain a volatile oil called allicin. Allicin is an oxidant that is, in part, responsible for its beneficial effect. The fresher the allicin, the more toxic the oxidant is.
Topically, fresh allicin causes a chemical burn and, in severe cases, can blister the skin.
Feline hemoglobin is very susceptible to oxidative damage and allicin oxidation can cause a severe and fatal Heinz body anemia in this species.
Chronic use of garlic in canines can cause anemia as well. However, depending on the dose and size of the dog, it is less likely to become fatal though it may persist for a period of time. Fresh garlic on its own can burn a dog’s mouth, esophagus and stomach. Lower doses can cause nausea, sweating, fainting and redness, while higher doses can cause ulceration of the esophagus and stomach leading to bleeding.
The fresher the allicin, the stronger the medicinal benefit but the more severe the adverse effects.
The older the onion or garlic, the lower the allicin content, the less medicinal benefit, and lower adverse effects.
To conclude—allicin in any form can be fatal for cats. Allicin for dogs, cautionary use.