Comfortis is great! I have been using it for 6 months now. I live in Florida and fleas are out of control, since I have been using it, QTIP has not had one flea. I highly reccomend it. He has had no side affects. Also I have had no fleas in the house. I have a himalayn, so is so allergic to fleas that she pulls her hair out, since I hve been using comfortis on qtip ther hasn,t been one flea Andrea