I had a case last year of a woman in California who was rotating six different kinds of Pharmaceutical Antibiotics and antifungals, in addition to dozens of dietary supplements. These cases are not as rare or isolated as you think, we see them all as practitioners, especially when you see chronic cases.
Yes, all those fillers, additives, etc. Many dietary supplements today are suspect and of dubious quality. I've been formulating for almost twenty years and can tell you that most companies use budget raw materials and then glam up the label, making all kinds of ridiculous claims. Marketing is everything.
Ask them where their raw materials come from. Hint: the same place that just has a big earthquake in the N/Hemisphere. Unfortunately, the standards of manufacturing are not that great over there, very poor if any GMP and zilch quality control to speak of. I prefer raw materials from Germany, Switzerland, Japan and USA myself, and when you make your own stuff you can specify where the materials come from.