I didn't see any proof with fleas either when I gave ours garlic capsules for a time and they didn't get ill either but somebody I know said that their rotties got sick when fed a bit of garlic for flea prevention. I think some dogs are ok with it and others not.

I know onions are supposed to be more toxic to dogs than garlic in any form, raw or cooked so you'd have to be more careful with those.