Chemically equivalent but formulated differently. Think hot dog vs. corn dog: the meat is the same but the packaging is slightly different.
The packaging can have a surprisingly large effect on drug absorption, and therefore efficacy. Notably with difficult to dissolve drugs a lot of work typically goes into formulating them to be more efficiently processed by the intestinal tract.
The FDA approves generics based on bioequivalence, not just chemical composition. In order to be an "AB" rated generic (and used as a substitute for the branded product), the generic has to fall between 80-125% bioequivalence of the brand, which measure dissolution and absorption.
The fillers in the generic don't have to be the same as the branded product, but the drug does have to behave the same as the branded product.
There was an issue I read about with a drug where they did not test all the different dosages of a generic, and I think the behavior of one of the doses was not as expected because it didn't dissolve well or something.
This was a psychiatric drug, and so of course, reports of problems went unheeded for years because patients were assumed to be imagining things.
Saying "the drug does have to behave the same as the branded product" is like saying "the product as delivered does have to match the spec".
We don't know that "it works in nearly all the cases".
We know that in nearly all the cases, any issues don't rise to the level where the FDA or the public identifies them. I doubt that the system is constructed in such a way that nearly all the problems are identified.
It seems to me that because drugs are developed to meet the absolute minimum standards for proving efficacy and safety, there's inherently going to be little if any margin for error in the manufacturing process and hardly any way to analyze problems. When reporting potential side effects, most drugs are going to be just above the noise level in their principle effect, so how are you going to determine what is a random unrelated issue and what is a negative side effect, and what is the consequence of a QA or design failure?
The packaging can have a surprisingly large effect on drug absorption, and therefore efficacy. Notably with difficult to dissolve drugs a lot of work typically goes into formulating them to be more efficiently processed by the intestinal tract.
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