Yuki, Ooops forgot to answer that one, or try to. that is a good one for Dr. Williams. I have four cats and have never had a problem, so I think it might not harm cats, but I would love to hear what someone else thinks. I gave the cats Colloidal Silver also during the out break here so that might have helped, it sure saved ll the ferrets. Stephanie [Posted in FML issue 1268]