I used program many years ago, then had a 14 yr span with no fleas. A few months ago we noticed fleas on our three cats.  The newest way to treat fleas is with topical applications, so we gave it a try. I hated it, and so did our cats. They smell and create an oily mess of their fur. I was also worried about them ingesting it because they groom each other. So I went back to Program.  You add it to their food once a month, they don't even know it's in there. It does take a couple of months to be fully effective because it doesn't kill the adults, just the eggs. I'm happy to say that I'm going on the fourth month of using Program and we have no fleas! I'm going to keep using it through the summer just to make sure new fleas don't find our yard!