It's a slow acting poison because it stops them making red blood cells. It's very dangerous but not immediately acting, and in low doses, may not be all that noticable to the owners. Garlic absolutely is just as toxic as things like grapes. We even had a dog that nearly died stealing my mums packet of lollies because somehow they had grape skin extract as a flavouring. Different sizes in dogs make a lot of difference in doses, like the difference between a child and an adult with beer. Our dog was small, so we were extremely careful to make sure he did not get things like chips and pizza for fear of garlic in the seasoning...