Fipronil has a very wide margin of safetylarge quantities of Fipronil would have to be involved to produce negative effects in dogs and cats. In present concentrations, we don’t expect toxicity problems from Fipronil or S-methoprene, an insect growth regulator that’s also present in many flea and tick products.
As with any flea control product, it’s important to consult with your vet to find the most appropriate program for your pet. It’s also essential to follow label instructions for correct use. For example, if the product is for dogs only, do not apply it to cats or other pets. If the label tells you to apply the spot-on liquid between the shoulder blades and at the base of the tail, don't place it in one spot, since irritation may occur. The opposite holds true as wellif instructions state to apply it in one spot, spreading the product in multiple places may reduce its effectiveness.
Like people, certain animals can have allergies or sensitivities to topical products. If your pet has preexisting skin problems or wounds, or has been known to react negatively to products applied to the skin, it's best to talk with your vet to make sure that a topical insecticide is the right course of treatment.