The "rumor" that Frontline has lost its effectiveness is just that....a rumor!
My Vet started using another product for flea and tick prevention. One of the marketing tools of the new product is to claim that Frontline is no longer effective. Frontline is still useful in flea and tick prevention.
I was never comfortable putting the whole tube of Frontline on Pearl, she is so tiny. But the Vet says that there is a huge safety margin. I still don't like doing it but the ticks are soooooo bad here in Delaware. Pearl ran into a nest of baby ticks, called seed ticks. She was covered with them!! It was horrible. Thankfully, seed ticks rarely carry Lyme disease or Ehrlichiosis. Pearly still had to have blood testing done to be safe.
As far as using Pet Meds, IMO, as long as the products are not outdated and they are in the orginial packaging, they are OK. Some Vets will NOT write scripts for Pet Meds. 1st off it cuts in on the profits of the Vet practice. Also, any med not purchased at the Vet is subject to improper dosing, etc.