Earlier quoted context omitted.
The big problem is that the pills people are obtaining are "just a horse dewormer" . The specification is different, the dosages are different, and the active ingredients differ (drug delivery is frighteningly complex area). Meanwhile people made enough run on ivermectin-based horse dewormer that people had trouble getting it for livestock.
> the dosages are different, and the active ingredients differ Neither the dosages nor the active ingredients are different. Both for humans and for animals, the recommended dose of Ivermectin to treat parasites is 200 micrograms per kilogram of body weight. The "active ingredients" are Ivermectin, period. I have a bottle of injectable Ivermectin intended for cattle here, and it says it's pure Ivermectin in propylene…
I think he might have meant the dosage that it is sold in is different.
Ivermectin for treating parasites is typically supposed to be taken as a single dose. For some parasites it might need to be repeated in 3 to 12 months.
If a doctor prescribes it to treat a human I'd expect the prescription to be for the right amount of pills for your body weight to deliver that single you-sized dose.
Horse ivermectin is sold in a quantity suitable for a 1250 pound horse.
From what I've read it appears that a lot of people buy that horse-sized single dose and use it all. Worse, a lot of them seem to take it daily.