My daughter took her new dog to the vet for a checkup, and to get heartworm preventative medication. She had used Revolution on her last dog, and wanted to use it on her new dog. They told her they were no longer selling it. That they had to many dogs testing positive that were on Revolution.
I asked my vet (different than the one she used), and he said he had quit selling it a couple of years ago for the same reason.
He does not know if its owner error or not, but decided the risk of heartworms to the dogs were to high.

You can still get it but the vets aren't making the money off it like they used to. The company keeps pricing it higher and higher and folks stopped using it. Revolution® Heartworm Preventive by Pfizer is still going strong! It's some of the best prevention out there.

One of the vets would still order it for the clients that did not want to change. They just did not advise using it. Both vets have their own personal practices, and are not affiliated in anyway. They are not even in the same town. One in Tomball and the other in Spring. These are the type of vets that have no problem with clients that use ivormec as a heartworm preventive. I don't think either would make up stories about it, just because they don't want to carry it due to higher cost.

Several years back, I got Pfizer to pay for the heartworm treatment for a client's dog that had contracted the disease despite regular usage of Revolution. Everything came out alright and the dog survived, but the client had lost all confidence in using Revolution.

Since the active ingredient in Revolution is selemectin, an ivermectin derivative, one would think that ivermectin might also be losing effectiveness. This doesn't seem to be the case.

My theory is that Revolution still does the job. Since it is a liquid that is applied monthly to the dog's back, I think many dogs are shaking the product off their haircoat before the it gets absorbed into the bloodstream. I believe that if the client will part the hair and apply it carefully to the skin, it will work just fine.

Their theory to, was that it was most likely owner error in the application.
Its something that they have no proof of, as a owner would have to be willing to admit it.
The easiest way for them to remove the possible error, was to stop recommending/selling it to their clients. You can't blame a vet that does not want the dogs in his care to test positive for heartworms.

I've used it for decades, all my dogs die of old age.