Generic medications refers to those drugs that are essentially carrying the same active ingredient as the original medication from which they are matching. If they have the same active ingredients, they should have the same pharmacological properties – and therefore should have the same dose, strength – and effect.
The great thing about generics is that they create more competition on the market and bring down prices (since the original patents fall through). Of course, pharmaceutical companies, for all the bad press, do a lot of good as well – the original drug discovery and clinical trials are all funded through them (and it’s usually a few hundred million).
So what if we took something important, like vancomycin, and compared the original vs the generic? This study from Antimicrobial Agents and Chemotherapy did just that, and showed that the generic vancomycin was significantly inferior to the original in vivo, even though in vitro they were pretty much the same.
Pharmaceutical equivalence may not always mean therapeutic equivalence – read more about it here.