I've used Trifexis since December on my female with no issues at all. It does NOT protect ticks or fleas though. Just the worms. I use scalibor flea and tick collar on her as well and haven't had any issues (but my pup did just have a tick, so i'm assuming her collar works) Trifexis is not expensive if bought in bulk. You can buy 6 months at a time. Individually it gets up there, my pup is in limbo of the puppy one and the 20-40lbs and it's about 20$ a month for a pill for him. Luckily, he should be 20 lbs the next time I need to get it. Also, on the topic of Trifexis being harmful.. it honestly depends on the dog. When I got my pup from my breeder/vet I asked her how early I could start him on heartworm medicine and if it was okay to give him Trifexis (since my female was already taking it) she said I can start asap on the pills and that Trifexis is really one of the few options remaining for shelties to take safely with no issues. That being said though, each dog is different and reacts differently to medicines.