Here is a very good example for you....
ibuprofen (generic, which is why it is NOT capitalized) is the same drug as Advil (and Motrin, and Excedrin)
aspirin (generic) is the same drug as Bayer.
naproxen is the same as Aleve.
Have you ever bought store brand ibuprofen?? By law it must have the exact same chemical components in the active ingredient. Therefore, 500 mg of ibuprofen is EXACTLY the same as taking 500 mg of Advil.
One way we mostly always get generics is with pain medicine. A doctor may write Vicodin on your script, but when you get it filled you may notice that the bottle says acetominophin (sp) and hydrocodone. This is because it was substituted with the "generic" at the pharmacy.
Let me clear one more thing up...
Generic drugs are not the same are generic foods ,etc (although this is a matter of debate

). That is to say, "generic" drugs are just NOT brand name. There are many companies that make metformin, but only one that makes Glucophage. It isn't so much that metformin is the generic of Gluc, but more like Gluc is the BRAND NAME of metformin (*because Gluc. IS metformin).
I hope this has made some sense!!
StaciieM.