Because the US has a significant attitude problem when it comes to international standards? Basically there's a committee that gives names to medical substances that are trademark free and can be used in scientific literature.
Internationally there's the International Nonproprietary Name (http://en.wikipedia.org/wiki/International_Nonproprietary_Name), given out by the WHO. The US just hast to do it different and they have the United States Adopted Name (http://en.wikipedia.org/wiki/United_States_Adopted_Name).
Probably a historic artefact, meaning sometime earlier every country had their own generic names, and adopted the INN, except for the USA (and Canada by virtue/vice of being in the same market).
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Internationally there's the International Nonproprietary Name (http://en.wikipedia.org/wiki/International_Nonproprietary_Name), given out by the WHO. The US just hast to do it different and they have the United States Adopted Name (http://en.wikipedia.org/wiki/United_States_Adopted_Name).
Probably a historic artefact, meaning sometime earlier every country had their own generic names, and adopted the INN, except for the USA (and Canada by virtue/vice of being in the same market).