Posted by Carlos (Lisbon, Portugal) on 08/27/2008
[NAY] I have done some searching online to find the pros and cons of Garlic and pets. I have come across a few links that suggest that Garlic can be fatal to pets - see below;
Garlic is part of the onion family (alliaceae) along with leeks and shallots. There is ample research available which indicates onions can be harmful, if not deadly, to our pets. In the last five years, more and more toxicity studies are being conducted on garlic and all seem to indicate that it, too, can pose serious health risks when fed to cats and dogs. A 2003 study on Grape and Raisin Toxicity in Dogs, published in the Australian Veterinary Journal begins, "The list of commonly available human foods toxic to dogs continues to grow. Grapes and raisins can be added to onions, garlic, chocolate, and macadamia nuts as posing dangers when ingested in excessive quantities."  Unfortunately, no one knows what constitutes "excessive quantities".
In an article on Onion and Garlic Toxicity in Dogs and Cats, Jennifer Prince, DVM states: "Garlic and onion are used as flavor enhancers in food. Since the toxic amount is unknown, it is recommended not to add it to your pet's food. These ingredients can cause Heinz body anemia, resulting in a breakdown of the red blood cells and anemia." 
Although the exact toxic dose is not known, studies unanimously agree that foods containing garlic should not be fed to dogs.
Full report available here -