We switched to using Trifexis at my previous clinic, and my current clinic uses their brand of it (same product, just under a different name).

Just like with ANY medication, you are going to see some type of reaction on a large scale. In the 3 years we've been using it at my clinic, the ONLY complaint we've had was vomiting. 9/10 the vomiting was because the owner did not give the pill with a meal.

It is perfectly safe. I use it on my own dog with no adverse reactions. Any "new" product is going to have a lot of uproar over it at first until it's established on the market. I like it a LOT better than other combo products on the market. Separately, the products are interceptor and comfortis... so look up reviews for those products separately and I'm sure you'll find more good than bad. It's just because they've combined it "recently" and put a new name on it so people are being overly-cautious with it.

EDIT: For clarification, Trifexis is not just a heartworm medication. It is also your monthly flea control but it is NOT labeled to work on ticks (there is some talk that it may have some efficacy against ticks, but the data isn't successful to the point where they can put it on the label.)