Sam is our 6 yr old cat. Very healthy and active. We always give our cat Advantage flea medication but ran out. My husband went to the store and got Harts flea medication to round up the year. The day after we give Sam the medication, he became very slughish. Like he had the flu. We waited a day or two and he stopped all his functions. We took him to the vet and $500 dollars later, they could only tell us he had an infection that effected his liver with symptoms like hepatitis. They had no idea where it came from. We are glad to say he is well again. We have Hartz to blame for a loss of $500, days off from work and a potential loss of our cat if we had not taken him to the vet so soon.