Depends on the drug; sometimes there are chemical differences, sometimes there aren't. Presumably there are also differences with respect to quality control an

Depends on the drug; sometimes there are chemical differences, sometimes there aren't. Presumably there are also differences with respect to quality control and regulations, at least in some countries. But in general if the drug has a single active ingredient then it should be pretty much interchangeable. In a couple of cases veterinarians have specifically advised me to buy certain antibiotics (for a sick dog) from the "people pharmacy" because it was actually cheaper than the veterinary version sold at feed stores. In the case of ivermectin tablets, usually available in 3mg, 6mg and 12mg versions both for dogs and for humans, there is unlikely to be any difference, and where I live the price is pretty much the same too.