If you MUST, and I am clarifying with a "MUST" Worm 2 times per year, Ivermectin has a proven Short Shelf Life once inside the Body.
And that is for Horses.
People are using smaller doses and means need to do it more often.
And what happens if a human uses Ivermectin alot?
You can also overdose on ivermectin, which can cause nausea, vomiting, diarrhea, hypotension (low blood pressure), allergic reactions (itching and hives), dizziness, ataxia (problems with balance), seizures, coma and even death. [7]