Can you use frontline and a flea collar?

No. Frontline products should be used on the specified species.

This is because one of the main components of Frontline, S-Methoprene is needed in a higher dose in cats. This is because it does not last as long in a cat than in a dog. So, if you used Frontline for dogs on your cat, it may not be as effective.

However, this higher dose is still safe to use on a dog - so you could use Frontline for cats on a dog.