A huge chunk of veterinary pharmacology is dedicated to getting you to NOT use cheap, over-the-counter flea, tick and heart worm treatments like simple pyrethrin-based shampoos (pyrethrin is so safe it is FDA-approved for food plants) and low-cost ivermectin.
To be clear, I am OK with folks using whatever they want, but I always advise caution with newer branded medications, whether for humans or for dogs. Cox-2 drugs like Vioxx have not proven more effective than Cox-1 drugs like aspirin, but they did leave over 20,000 Americans dead. Whoops!
The latest heads up in the world of dogs is Trifexis, a two-year old flea and heart worm preventative that is already linked to 7,000 dog deaths and an estimated 30,000 illnesses. Do these numbers mean Trifexis is the culprit, or that Trifexis is going to harm your dog? No. Remember that all animals present with a wide variety of reactions to everything, and as a consequence honey bees kill more people in this country than terrorists. That said, is Trifexis a medication I would stay away from for now? It is.