The Ivemectin for livestock contains more than just ivermectin. The syringe used for horse is enough to treat a 1250 pound animal. Each tick mark on the syringe is good for 250 pounds of animal weight.Thats a bit much for a human.
Equine drugs are as pure and human medical products. My vet friend said once of animal steroids, you kill someones million dollar race hourse and you can bet your behind you'll be sued, so Nandrolone for horses theoretically could be used in humans.
That's an oversimplification.
Apart from the medically inactive paste/flavoring, some horse pastes do have extra active ingredients and those should not be used by humans. Others only have Ivermectin, and they are safe for humans. The active ingredients are always marked right on the package.
The plungers in the horse paste products do have tick marks for 250 pound increments, but they are also subdivided in 50 pound increments, and a locking device on the plunger allows its movement to be limited. So, you can set the dosage limit for 50, 100, 150, 200, 250, and any other weight up to the maximum.
It's not idiot proof, but it is idiot resistant. The ability to read, count, and do simple arithmetic is required which leaves out many publik skool graduates.