Hi, I'm not familiar with Trifexis, but have heard lots of dogs have gotten sick from it. We use Revolution, which covers heart worm, fleas, ticks, and other parasites. Our guys have been on this for two years and have never had any bad reactions or side effects with it. It comes in a vial and it's applied to the base of the neck once a month between June and Nov. If Trifexis is making Leo sick, or he's having a bad reaction to it, you might want to try something else. I hope he's okay.