First off any of the flea and tick chemicals are very harmful to pets, and don't work as they should anyway.
Next, I find that dogs who eat dog food are the ones who are at risk for contacting fleas and a slew of medical problems.
I have two 5 year olds and one 2 year old that have never had a flea and have never had any medications. I'm into holistic, not chemical.
I home cook and I also give them HEALTHY POWDER, which has yeast flakes.
Here is some ideas at keeping fleas off:
Chinaroad Lowchens of Australia - Ticks, fleas, bees, flies, snakes and other gremlins! And how to deal with them!
I very highly recommend a book by Ann N. Martin, FOOD PETS DIE FOR. She's had the dog food companies in and out of court for over 7 years. She has proof and pictures on the rendering plants. Not a pretty picture to say the least.