Eeep! That is what I use on my cats! If it doesn't protect against heartworms (my number one concern as it is abundant here.) then I need to switch over meds and stop paying ninety bucks a month for six month supply (three, in all actuality, because they both get a seperate package each.). BUT I have never had a problem with it as they have been ahem *accidentally* let out a couple of times and have never shown a problem. I don't know. Maybe it is best to look into different things.