There is no scientific evidence that garlic is harmful in dogs, but I wouldn't advise giving too much. I've fed garlic to dogs for over 50 years without a problem. Now though I have small dogs, so I throw in a few fresh whole garlic in the rice, when I fry up the round steak and such, but I discard the garlic when the food is done. It's the oil in garlic that has all the benefits including an important aid in digestion. I also give what's called Healthy Powder from Dr. Pitcairns book and it has yeast flakes. Mine have never had a flea or tick. Here is the holistic and safe way to keep them off as well:
Chinaroad Lowchens of Australia - Ticks, fleas, bees, flies, snakes and other gremlins! And how to deal with them!