I have two puppies that are now between 4 - 5 months old. All of their shots are up to date, daily dog vitamins etc. I was thinking of ptting them of Frontline through the vet. Does it work really well ? Is it true that the fleas can actually totally burrow completely under the skin of the dog ? I knew that they will bite them and are blood drinkers, but I had no idea that they could completely burrow under the skin . Oh well ! What do y'all suggest?