Not all flea preventatives are created equal.  There are quite a few flea prevention products that we see on the shelves of pet stores and advertised through on-line pharmacies.  Some cost more than others and some require a veterinary prescription.  In my opinion, the primary and important difference in flea prevention is in “speed of kill†– allow me to explain what that means.  The various topical flea preventatives that are commercially available all have different time frames in which they are anticipated to kill the fleas that jump on the treated pet.  This is important because when we take into account the flea’s life cycle, we can understand that we want to kill that flea before it has the chance to lay eggs that will further contaminate the environment.  A topical flea preventative that kills the flea faster will act as a treatment for fleas (the flea is dead) as well as a preventative for fleas (the fleas are not laying eggs that will contaminate the home/yard and serve as a source for further infestation).  This combination of action gives us the best possible prevention and treatment for fleas. 
There are some other variations between the products.  Safety for children is an important one – in almost all cases it is important that the product not come into contact with humans.  Some are labeled as more harmful than others, so each situation must be taken into account.  My recommendation has always been to apply the topical flea product at night right before the family goes to sleep.  This way, no one will be petting the pet immediately after application and by morning, it will be fully soaked into their skin.  Tick protection is another way in which the commercial products differ.  I recommend tick protection in the topical flea preventative for dogs, not cats.  This is designed to kill ticks that jump on your dog, hopefully before they are able to transmit disease.  Ticks carry many diseases, the most commonly known is Lyme Disease.  If the tick dies quickly, it is unable to transmit this disease to the dog through the bite.  Finally, water resistance varies from product to product.  In most of the veterinary –recommended products, water resistance is an added feature.  Once allowed to dry, the product is resistant to washing off for dogs that swim or are bathed frequently.  
Please check the various products and their claims before deciding that price is your only concern.  I have seen many dogs with live fleas that were recently treated with an inexpensive over-the-counter product that their owner bought in the grocery store.  These products will appear to be no different than their more expensive counterparts, but they are!
Finally, please remember to use the product exactly as it is labeled and on the proper pet.  Dog flea products on cats can be toxic, as can cat flea product on rabbits.  

Dr. Amy Hellard
West Chester Veterinary Care (WCVC)