Honestly, I doubt that the use of brewer's yeast helps repel fleas. I could imagine why fleas would not like the blood of an animal that has consumed garlic, but I'd be curious to see research on how well Brewer's Yeast repels fleas. I stopped using topical flea medications years ago, but I do include a bit of garlic powder every now and then. That, plus the fact that our enviornment is very low risk, helps keep my dogs flea-free. With so many dogs coming and going, however, I think you shouldn't completely throw away the flea meds just yet. The risk is just too high :/