Just stopped working
I have used Frontline Plus for many years, and it worked great.........until this summer. I noticed fleas on me mutt three months ago, and it got progressively worse. I even started to apply Frontline every three weeks rather than once a month, but that had zero effect. It got so bad that I went to my vet and she stated that nearly all her patients that used Frontline were experiencing the same problem. It seems that fleas have become immune to Frontline. She recommended Trifexis (you have to get a prescriptions because it also helps prevent heartworms), and within about an hour no more fleas.