I do research at one of the major drug companies. They put all kinds of excipients in the tablets mostly to prevent the pill from crumbling or absorbing too much moisture from the air in humid climates. Sulfates, phosphates, silicon dioxide, all kinds of random things they use. Once it gets into your intestines the insoluble stuff just sits there. Your gut is very good at absorbing the drug itself out of all of the stuff in the tablet.