Rocky was on Trifexis by his previous owner, With no problems, But she expressed reservations when she explained it to me. Rocky is allergic to fleas, and was on medication for it, When I talked to my Vet, I expressed serious reservations when he recommended Trifexis, as being the best possible solution for both Heart Worms and fleas in my area. Rocky's records indicated he was on a few different Heart Worm preventatives in his past, My Vet also did some research, and his conclusion was< Rocky's been on this medication with no ill result, therefore why change it. My explanation of Bull Dogs dying because of it was of interest to him, But went further into contacting several other Vet offices in my area, each of which regularly prescribed Trifexis without incident.