I think if fipronil is taken in right amount as per weight of dogs according to prescription, it is not harmful. Basic prescribed is based on individual pipettes that are available for dogs in weight categories up to 22 pounds, 23-44 pounds, 45-88 pounds, and 89-132 pounds; over 132 pounds use the appropriate combination of pipettes or pipette plus spray. The side effects seen are temporary redness or itching at application site. The pet may drool if he licks the product (spray) before it dries.