Many products are not safe... like Hartz, and made my cat sick.  Advantage works great on all my cats, and they are  indoors at night and out playing most of the day. I alternate cats when I use Advantage, and apply it only every 3 weeks to ONE cat at a time in the winter.... still no fleas! In the summer I use it every 4 weeks as suggested. My daughter has 8 cats and a dog, and uses only Advantage for flea control.  It works better than anything else out there.  It's so nice not to have flea bites all around my ankles, or fleas in my home !!