Firstly, don't immediately panic - for the first week or so, it's possible large numbers of fleas will jump onto your pet. They'll touch the fipronil and become poisoned, but they don't die immediately, so you might see slow-moving or debilitated fleas on your pet for some time. It's important to understand that even with the best flea product, it can still take 12 weeks to get rid of an infestation due to the nature of the flea life cycle. That said, some people have reported resistance with fipronil-based products. If you are still seeing live fleas on your pet several days after treatment, you should look at changing your flea treatment. Your vet will be able to recommend a product, but we explain the options currently available in our free online resource.