The Dilemma Of Inherent Aggression
Big hot topic, but after my ordeal, only reinforces my belief. Here is a lady stating gave her dog Comfortis and moving….. then became aggressive. What is that?
Aggression from Comfortis
02/01/2016 12:34pm
I have 2 dogs, an American pit bull and an Australian kelpie. A few months ago they got fleas (after moving from AZ to MO) and we had gotten some comfortis from the vet to help. After two months, I realized something was wrong. My docile pitty (she has always been a lover, not a fighter) started fighting with my mom’s dog as well as her mate. This is highly unusual and was worrying me. The only change in her diet was the comfortis and ACV. We took into account the logistical change but she has been out here before and it was never a problem.
She has gotten into 3 fights now, each time was when we were all playing and all the dogs were running around and energetic. After the fight was over and everyone calmed down, she would go into this almost remorseful curled up ball in her kennel, and we noticed her giving little doggie kisses to the other dogs with this incredibly sad look on her face (adorable, but seemed almost apologetic). We are letting the comfortis run its course out of her system, and then switching to more natural
Louie's Law 
Comfortis has no known side effects of aggression.
http://www.petgroomerforums.com/chat/archive/index.php/t-22584.html
However there are other side effects it does induce. One is seizures.
Many pitbulls seem to turn at some point. By nature they are inherently aggressive. They have a high predatory drive to other animals. They were ultimately bred for taking down bear and boar.