Some people have disagreed with me when I explained about the 20% plus or minus difference that the FDA allows for generic drugs, some have insisted this was only for the inactive ingredients. So I have done some more research on this subject, and NO! the 20% difference IS NOTjust limited to the inactive ingredients.

Here's an article with some information, and the name of the law that allows this:

http://counsellingresource.com/medications/discount-drugs/generics.html

*"Are There Any Differences Between Generics and Brand-Name Drugs?

... it should be noted that current regulations permit a variation of up to 20% either way in the bioavailability of the active ingredient. (See this Food and Drug Law Institute overview of the Hatch-Waxman Act of 1999.) In one study (Borgheini 2003), a 31% variation was found in the blood plasma levels of a particular medication after a patient switched from a branded to a generic product. (Why does this happen? It may be accounted for by differences in the manufacturing process yielding different particle sizes that are absorbed at different rates, as well as other factors.) "*

So yes, this gives them a leeway of 40% on manufacturing these drugs, they can be significantly different than their name brand counterparts, and in some cases the effect can mean a drug doesn't work at all, or may have too much of the active ingredient, resulting in serious effects or even fatalities!
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Here's the overview of the law in question that allows these differences:

http://www.fdli.org/pubs/Journal%20Online/54_2/art2.pdf

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For some of you, I know this will be a relief as you can now know for sure that if a generic seems to work different, it is not all in your head. Simply said, poor quality control and poor testing, means poorly made drugs from some companies.

My information is not guaranteed correct. I do not get them right all the time, but I do enjoy the hunt~