If in doubt, always call your vet or the Animal Poison Line immediately.
Garlic contains the same toxic compound as onions (thiosulphate) but at 5 times the concentration. Garlic powder is especially dangerous as it is far more potent than raw garlic. Garlic-flavoured dog treats and homemade garlic bread are common accidental sources.
Call your vet. Garlic poisoning may not show symptoms for several days as red blood cells are gradually destroyed.