A couple years ago, on a Black Friday sale on Amazon, I got sold counterfeit collars from Amazon directly. I didn’t realize, until a couple months later when I saw an article and checked the collars, that they were counterfeit. They had not been used and were still in the cellophane wrapper in the tin. I had to tussle with Amazon a bit about the refund, but since they were the actual seller, I finally did get a full refund.
That being said, I now get the collars from either my vets office or Chewy for our two dogs plus my neighbor’s dog. We have never had any issues with them not working. I had used Frontline Plus for years, but started finding ticks and fleas on the dogs.
But, this thread is a great reminder- anytime you use any sort of chemical, drug, vaccine, etc. there is a risk involved. Bodies can have reactions. Sometimes they can be extreme, and sometimes even fatal.