Can you use frontline for cats on a dog?

The general advice for this is no. The formulation for cats is different than that for dogs, and since cats are much smaller the total dose of active ingredients will be low, particularly for a large breed dog. The risk of underdosing your dog is this can lead to a false sense of security - you think your dog is protected from ticks and fleas, but he is not and suddenly you realize after a few days that your dog has a serious tick infestation or is covered in fleas. Also, underdosing can lead to the development of resistance among targeted parasites, which could cause the Frontline to no longer work for your dog.