Pyrethrins, which are relatively safe even on baby kits, act as flea repellents and kill adult fleas. Products containing pyrethrins and similar ingredients, such as resmethrin are available in many forms including powders and sprays. Use a product that is labeled for use in ferrets, unless your veteriarian directs you to use a product 'off-label' An off-label product is one that is not licensed for use in a certain species or for a certain condition, but may be prescribed for such use by a veterinarian, such as Advantage.