When we first got our dog, 4 years ago, we tried Hartz as a preventative for fleas. I still feel guilty. If I had known then that my dog would still have skin issues due to the collar we used, I NEVER would have even tried. Within 24 hours he was itching and biting so bad that I called the emergency vet to see what could possibly be causing his distress. After talking to her for less than 5 min, I could not get the collar off of my buddy fast enough. I know my dog has forgiven me, but I will always feel guilty and ALWAYS tell people NOT to use any Hartz product.