Your question really peaked my interest. I often take generic versions of both prescription and OTC drugs, usually without problem. But a number of years ago I was curious as to why my Dr prescribed the cheaper generics for most of my meds but not for one of them.

She told me that although the active ingredient and a name brand and its generic are chemically identical, they are not always the same in how they are released into our system. In many cases, she told me, that isn't important but sometimes it is. Some meds require an even release over time. For example, both the brand name and the generic of drug "X" may both contain 20mg of chemically identical drugs (as required by the FDA), but the brand name will release that 20mg at a very steady rate over several hours where a generic may release it unevenly or even dump all 20mg into your system at one time.

I've always been curious as to why that might be. Your question motivated me to do a bit of research. Here is what I found in the Merck Manual:
--quote-- Although 250 mg of a trade-name chemical is identical to 250 mg of the same generic chemical, a 250-mg generic pill containing that chemical may or may not have the same effect in the body as a 250-mg trade-name pill. That is because everything that is used in a particular product formulation affects how it is absorbed into the bloodstream. Inactive ingredients such as coatings, stabilizers, fillers, binders, flavorings, diluents, and others are necessary to turn a chemical into a usable drug product. ---/quote---
http://www.merck.com/mmhe/sec02/ch017/ch017b.html

The take away is that although the active ingredients are identical in the name brand and the generic, the inactive ingredients may not be, and sometimes that makes a difference in how they work.

I also found this very interesting (from Columbia University):
--quote-- Bioavailability is the amount of time the drug takes to be absorbed into the body under identical circumstances; manufacturers of generic drugs must show that the bioavailability of their product does not differ by any statistically significant amount (often considered 20 percent from the mean absorption) from that of the brand-name product. Generics have the same amount of active ingredient; the amount of time it takes for your body to absorb it may be slightly different, but not by enough to change the effectiveness of the drug. ---/quote---
http://www.goaskalice.columbia.edu/2547.html

In other words, when the FDA says that the bioavailability of brand name and the generic are identical, they don't actually mean identical. They mean they are pretty close. In most cases, close is good enough. But (again quoting the Merck Manual)
--quote-- Drugs that must be given in very precise amounts are less likely to be interchangeable, because the difference between an effective dose and a harmful or an ineffective dose (the margin of safety) is small. ---/quote---

Two years ago, ABC News did a story titled "Generic Drugs: Dangerous Differences?" that investigated these very issues.
http://abcnews.go.com/Health/Depression/Story?id=3731555

It's interesting to note that in this report it pointed out that sometimes patients actually do better on the generic.

I guess the bottom line is that generics are usually just fine (and definitely cheaper) and occasionally even better, but you need to pay attention to your body's response and be prepared to stick with the name brand if the generic doesn't work for you.

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