I am not an expert on this subject because I buy dog products for my dogs and cat products for my cats and I only buy them from my veterinarians because neither of them will provide written prescriptions for mail order purposes. They both are so reasonable on our vet charges, I don't mind them getting the up charge on the heartworm and flea meds. But, if I'm not mistaken, there is a major ingredient difference in the topical flea meds given to dogs versus cats and the dog meds include something that could be toxic to cats. I'm sorry I don't remember the specifics, but I would not risk using a dog flea treatment on my cats. I'll try to get the specifics and post again.