Eli5: what is the difference between a generic drug to the original drug, and why do some doctors will swear by the original drug?
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I believe there is one factor that these other answers are missing that makes generic drugs chemically different (sometimes) from the original drug.
When building large molecules, often you get both one version of the molecule (call it Version1) along with its mirror (Version2). For many drugs, which of these versions you get doesn’t make a difference. However, sometimes only Version1 of the molecule is an effective medicine. Also, sometimes Version1 and Version2 have side effects with different levels of severity.
Different ways of building the molecule give different amounts of Version1 relative to Version2, and the original manufacturer keeps their manufacturing process secret. So, if the generic is using a different chemical reaction to build the drug, and it ends up creating more of the inactive version of the drug, it will be less effective despite being technically the same medicine.
This is a bigger problem for some drugs than others and some people than others. Doctors know this and so might stick with the name brand for some drugs and be fine with generics for others. Notice this gets more complicated when you realize that a drug might have several generic manufacturers each using a different process so all their drugs contain different amounts of Version1 and Version2. So it’s not necessarily the case that the generic version of a drug sold by one pharmacy is the same as the generic version of the same drug sold by a different pharmacy.
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