My English Setter had fleas for the first time this year. She always wears a flea/tick collar. I had the Hartz 3 in 1, applied it per directions. I noticed the next day she was very timid and would start shaking if I approached her. Over the next 2 weeks she stopped eating and was having diarrhea. When we brought her to the vet she was weighted and had lost 12 pounds since her yearly physical only 3 months earlier. She was put to sleep that day.