Frontline is completely safe to use on puppies 8 weeks of age and up. Use the WHOLE thing, even though it seems like a lot. My little girl is 9 weeks and I just noticed today that she has picked up fleas from somewhere, probably outside. So, I just took her to the vet on my lunch break bc I didn't think she was old enough or big enough (only 1.8 lbs) to use Frontline on her. But, the vet said it was perfectly fine to use the "Frontline for dogs 22 lbs and under" on my little girl. She said to use the whole thing.
