Many want to know dosages to use and other treatment information. Ivermectin has been around since the early eighties. But when it does, we can show neurotoxic signs because GABA is also important in our brain tissue. Neurotoxicity in mammals can include listlessness, lethargy, uncoordination, disorientation, tremors, pupillary dilitation, weakness, agitation, blindness, head-pressing, and death. So much for the scary part.
Many veterinarians use ivermectin for a variety of parasitical conditions with great results.
I will not go into the large animal use except to say it is widespread and because the animals are bigger, the dosage forms are more concentrated than is ideal for dog and cat use. Off-label use of drugs in veterinary medicine is extreme common, thank goodness. If we had to rely on just the small set of drugs properly approved by the FDA we would be in a tight spot much of the time. Nonetheless, ivermectin is a drug that can have serious side effects, and should be used only under the guidance of a veterinarian with full disclosure of it's off-label usage. The very low dosage used in the monthly HW pills is sufficient to kill any heartworm larvae a dog may have acquired from a mosquito bite in the previous 4 weeks.