Dogs should not eat garlic. In a sufficient amount, garlic and other vegetables in the same family, including scallions, onions and shallots, can damage a dog's red blood cells. Damage from consumption of garlic may not be apparent right away, according to the ASPCA.
Garlic is more toxic to dogs than onions, and although it is rare for dogs to consume enough of either to cause a serious issue, concentrated garlic or onions in the form of garlic powder or soup mixes can cause a problem if ingested. Symptoms of a problem may only occur three to five days after consumption when the dog is lethargic or tires easily. The dog's urine may be dark red or tinted orange. Veterinary evaluation is necessary if the dog shows symptoms of toxicity, advises the ASPCA.