The scientific method generally doesn't prove negatives. In this case, a trial is done where one group is given ivermectin and the other is given sugar pills

The scientific method generally doesn't prove negatives. In this case, a trial is done where one group is given ivermectin and the other is given sugar pills (placebo). Their health outcomes are compared. If you have enough people in the trial and there is enough of a difference between the outcomes of the 2 groups then you say you have evidence that ivermectin is effective. The well run trials to this point don't show a significant difference between the groups. The ivermectin advocates say that the trials have not been run correctly for various reasons like dosage, timing, etc. More trials can be run to try to address these concerns. This can be repeated any number of times but at some point people will give up trying if no effect is found.