I don't see how parasites could develop immunity. Certainly bacteria cant so that should apply to parasites too. I think some stomach problems could be caused by irritation to the stomach wall by too high concentrtions or not enough dilution
so those with sensitive stomachs should take care and use lower doses perhaps more frequently and if you get stomach discomfort try and find a level where this doesnt occur. You can also take herbs to soothe the stomach if this occurs.
Dr Charlie is now just plain Charlie btw