Garlic is considered to be less toxic for dogs than onion when used in moderation.
Lee et al. (2000) studied wether dogs given garlic extract developed hemolytic anemia. Garlic extract was administered intragastrically (1.25 ml/kg of b.wt. (5 g of whole garlic/kg) once a day for 7 days).
Compared with initial values, erythrocyte count, haematocrit and hemoglobin concentration decreased to a minimum value on days 9 to 11.
Heinz body formation, an increase in erythrocyte-reduced glutathione concentration, and eccentrocytes were also detected, however, no dog developed hemolytic anemia.
Eccentrocytosis appears to be a major diagnostic feature of garlic-induced hemolysis in dogs (Lee et al., 2000; Yamato et al., 2005).
The conclusion says not to feed garlic: http://www.ncbi.nlm.nih.gov/pubmed/11108195
Garlic does not help to eliminate worms, neither does garlic prevent of ticks or fleas. However, garlic has benefits.
The Veterinary University of Zurich (Switzerland) advises not to exceed 4 g of fresh garlic per day for a medium sized dog.