I ordered Heartgard from this site this summer. It was shipped very fast, and the product appeared to be identical to one I have used before many times. All labels/stamps/seals/dates were present, nothing would suggest it might be a counterfeit. Per label, made in the U.S. I administered it to my dog twice.
Both times he had a seizure shortly after that. I had used this product before many times, without any adverse effect. So after the first incident I had no clue that these two events might have anything in common. After the second one I am convinced the seizures are result of my dog taking the drug. I cannot say for sure if the product is counterfeit. If it is, it is a highly sophisticated one. It could be a manufacturer's problem, too.
I contacted Merial asking to investigate. They refused to disclose where the certain batch was sold to. They claimed they have no such ability. They also stated that this site is not on their list of approved distributors. They promised to report the event to FDA. Otherwise, they showed no interest to investigate the origins of the box. I requested this info from the seller and will post their reply - if and when I get one.