Over the counter flea and tick preventatives like Zodiac and Sentry are very dangerous! They have been know to cause death! They are especially dangerous if your are using them along with Frontline. Just do a Google search to find information about deaths, lawsuits, etc. as a result of these products.
HartzVictims.org - Zodiac Flea Spot Treatment almost killed both of my cats! 
Fleas and ticks can become resistant to certain flea products in your area with constant exposure. Your vet will be able to recommend a safe alternative.