Tried the whole garlic thing. Didn't work as well as I hoped it would. As soon as I took him off Advantage, the fleas came back within a few weeks to months. While I'm a firm believer in natural diets and such, I honestly don't believe one single supplement like nutritional yeast or garlic can prevent fleas entirely. You could focus on improving the immune system as a whole which has been suggested to repel against fleas, but who knows? It's all up for conjecture.