Brewer's yeast is touted as a natural remedy for fleas for dogs and cats because fleas have an aversion to thiamine, which is found in abundance in brewer's yeast. Thiamine is part of the B complex vitamins. When you feed brewer's yeast to your pet, it ingests the thiamine and exudes it through its skin; it can even be found in its blood, and the taste and odor drive fleas away.
The recommended dose of brewer's yeast to drive away fleas is 1 tsp. for cats and smaller dogs, 2 tsp. for medium-sized dogs and up to a tablespoon for large dogs. It will take at least four weeks of oral doses of brewer's yeast for the brewer's yeast to effectively drive away fleas, so you should begin supplementing your pet's diet with brewer's yeast in early spring if you hope to prevent a flea infestation. Brewer's yeast as an oral supplement will not work as a flea deterrent for every pet, though it will work with most pets. While excess B vitamins will be flushed out of your pet's body in its urine, excess amounts of brewer's yeast can cause skin allergies. Keep an eye on your pet's health and do not give it excessive amounts of brewer's yeast.
If you are unsure about feeding your pet brewer's yeast or want to speed up the process of driving away fleas, you can also apply brewer's yeast externally. Put brewer's yeast on your pet's coat and rub it in. Not only does this drive away fleas, but it is also much safer for your pet to lick brewer's yeast off of its coat than to lick and ingest the pesticides found in most flea powders.