The challenge is toxicity. Toxicity can come from taking too much in a single dose, but the bigger issue is the cumulative dosing with taking the drug for multiple days in a row. Ivermectin is labeled for a single-use dosage in both humans and animals. Whether treating a person for river blindness or a horse for worms, it’s intended to be given once and not multiple times. It stays active in the body for a length of time. If someone takes ivermectin day after day or week after week, as one might with an anti-viral or antibiotic, the dosage builds up in the body to toxic levels. Even with the human form, if someone takes one dose for 10 days in a row, they end up with 10 times the recommended dose, which can certainly cause toxic effects and overdosing.