My read of the studies is that Ivermectin might be helpful when used early after or even before exposure. Benefit increases with dose but so do side effects. If you use it every time you are exposed, you may end up overdosing indeed. Positive studies on the site are small, large ones are negative – claimed to be done under bad protocols.
This is how it should work – after small studies show promise, you organize a large study that follows a promising protocol. People who are promoting this should focus on creating such trial. Given the cost of the drug, this would be an easy one to do.