Not so much the rabbit as the protozoa, it may become immune which would then be very dangerous indeed. It may also damage any natural immunity the rabbit had developed to it by reducing the antibodies produced, thus leaving the rabbit more exposed.
I also don't know what the longterm side effects could be over a lifetime, I don't know if any studies have been done into this. Just because Panacur is safe over prolonged periods dosn't mean it will be safe to use indefinately.
