If you took a cheap, store brand
aspirin tablet and an expensive, brand name
one, and put both in 2 separate glasses of water. The brand name will probably dissolve within seconds, which is what you want, since aspirin is very acidic. But many generics will just sit there....barely dissolving at all. The side of that water glass could be your stomach, and the cheap aspirin will be eating a hole in the lining, or at the very least, causing irritation.
Another example: Thyroid meds.