answer:Frontline is applied to the back of the dogs’ necks and does not spread to the rest of the house. All adult fleas currently on them will be dead in 24–48 hours. Then you have to be diligent with thorough frequent vacuuming and discarding the bag each time instead of waiting till it’s full. Otherwise it becomes a warm moist breeding ground for the next generation of fleas. Yes, it’s more costly but it’s only for a few weeks or months and is definitely worth it. The vacuuming is what will eliminate the residual eggs and larvae which have fallen off your pet. This prevents the need for the spraying and fogging. The few remaining that eventually hatch out into adults will need to promptly find a food source. They’ll hop onto the dogs and be soon dead. The Fromtline spreads through the pets’ skin (not the blood) so all it takes is contact for it to affect them. Wash all bedding in HOT water and use a fine tooth comb on the dogs toward the end of each monthly time period to get rid of new hatchlings and eggs. The Fromtline must be applied at least every 30 days since it contains IGR compounds which work on all stages of the flea life cycle. If you try to cheap out and lengthen the interval, it gives their numbers time to rebound. Do your dogs walk all over your neighborhood or are they in your fenced yard? There are non-toxic nematodes available to treat your yard. They feast on flea larvae. If your dogs go outside of your yard, it’s best to keep up the monthly Frontline until the first frost which kills them off for the season. Begin treatment next year at the FIRST SIGN of warm weather. NOT at the first sign of any fleas. Then you’re playing catch up. I have indoor only cats and am very sensitive to toxic chemicals like sprays, bombs etc. All I used was three months of Fromtline, religious vacuuming and combing and my cats and homr were TOTALLY free of fleas in 3 months. Fromtline is available through your vet. Don’t even bother with the Hartz and other OTC stuff. Never mind flea collars. All that is total crap and has proven deadly for SOME young or small animals. The state of the art for flea control nowadays is products which eliminate or weaken fleas at ALL stages of their life cycle. Go to Frontline’s website. They have a thorough explanation and excellent diagrams on exactly how IGR (insect growth regulators) work.