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How Does Flea Medicine Work?

| Updated September 26, 2017


Most flea medicines prescribed by veterinarians are topical fipronil-based treatments that are applied to the pet's skin. Fipronil is a solution that's used to kill fleas (and sometimes ticks) in animals. These treatments, which are usually repeated monthly, are applied to the area between the animal's shoulder blades. Because these solutions are poisonous if consumed, they must be applied to a spot where the pet can't reach it with its tongue.

Spreading the Solution

Once the solution is applied, it takes about 24 hours to spread across the body. Unlike flea treatments that are ingested, fipronil-based solutions do not enter the animal's bloodstream. The solution gets into the animal's natural body oils and uses the oils as a means to attach to the pet's skin and hair follicles. Because the presence of natural body oils plays a big role in the medicine's effectiveness, it's best to apply the treatment at least a few days before or after a bath. Once the solution is distributed, it releases toxins over the course of a month that make it impossible for fleas to live on the animal.


Fipronil-based solutions kill fleas by paralyzing their nervous system. The bugs become unable to survive on the animal, and don't need to bite for the solution to affect them. These types of solutions usually kill any fleas on the animal's body within 48 hours of application. As long as the solution is applied about once a month, the animal's skin remains an inhospitable environment for these parasites.