Since we work with a lot of large hunting dog kennels, a common request is for us to "mix up some heartworm stuff in the lab for us to use". What they want is a cattle dewormer, Ivomec, mixed in a vehicle for heartworm prevention. Ivomec's active ingredient is ivermectin, the same active ingredient in many brand name heartworm preventatives.
On paper, and in real life, this off-label product will, in fact, prevent heartworms. But, the legal ramifications for a licensed DVM to provide this off-label product is complex. First, the FDA allows veterinarians to "compound" drugs for special situations where there is no alternative. But, with heartworm prevention, there are many labeled products available for this use. As a result, the FDA looks negatively on mixing heartworm prevention in the back room just to save a client money.
Secondly, if an owner is using this cattle product on their dog to prevent heartworms, there is a myriad of legal issues if the dog either gets sick from the product or gets heartworms while on this medication.
As a result, we have quit providing this back-room mixture to our clients. The good news is that since the patent has run out on the ivermectin-based brand name products, the costs have decreased substantially.
If you own a large kennel, you may be tempted to get a friend or even your veterinarian to mix you up some of the cattle product to save money. I recommend that you be very cautious in this approach because you are out on a limb if problems arise.
Everyone likes to save money, including me. But, there is a false economy at times and, with heartworm preventions, you are truly better off using the right stuff.