Spinosad is one of the newer additions to flea control, milbemycin has been around for a long time. I am not uncomfortable with the combination and although there were reports of problems I do not feel that are warranted. I have used Frontline and Heartgard for years once the option of Interceptor (milbemycin) were no longer offered. Some of the new products like Nexgard and Bravecto for external parasite control have become available I am comfortable with them. Adding Heartgard for internal parasite control provides broad spectrum coverage.