Well you have to understand that there are tons of companies available that manufacture various generics, if a generic version has been available previously, this might not even necessarily be a new version of the med, just simply different from what you are used to getting.
And no, the formulary does not have to be exactly the same among generics. They can use different fillers and binders, as long as they use the same active ingredient, which is why some generics do not work well for some people, something in the inactive ingredients just doesn't agree with their bodies.
As a matter of fact, the active drug in generics, are allowed, under FDA regulations, to vary by as much as + or - 20%. They also are not required to undergo the same quality testing that the name brands were, they can simply get the active ingredient, make their version, and toss it on the market.
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My information is not guaranteed correct. I do not get them right all the time, but I do enjoy the hunt~