Hi and thanks for submitting your question.

There is a simple answer. One dose of medication will help but it will not resolve the problem. To treat fleas effectively, you must disrupt the life cycle of the fleas by using multiple doses.

A dose of Advantage will kill the adult fleas on your pet, and it also has some activity on the larvae, which is the stage before they become adult fleas, but it does not kill the eggs in your home. There are vastly more eggs in your home than there are adult fleas on your dog. The first dose kills the adults, then the eggs in the environment grow up and jump on your dog, and then the second dose kills them. In fact, some people find more fleas about 2-3 weeks after using the first dose because there is a large wave of eggs that hatch and jump on your dog. The third dose should eliminate the fleas . So the quick answer is the first dose is useless without follow up doses. You must complete the prescription to kill the infestation on your dog.

As far as the application of the product, it will make no difference whether it is applied in 1 spot versus 3 different spots along the back. We often apply it in multiple spots just to be cautious. If your pet has a skin reaction to the medication, which is rare, the reaction may be small in each spot rather than an incredibly intense reaction in one spot. I would recommend you continue to apply in multiple spots along the back.

I hope this helps.

Dr. Clayton Greenway