Just to follow up, the FDA does require testing to show that the generic has the same bioavailability as the brand name drug.
You can get the full details
here on the FDA website.
From the above referenced page on the FDA website:
MYTH: Generics take longer to act in the body.
FACT: The firm seeking to sell a generic drug must show that its drug delivers the same amount of active ingredient in the same timeframe as the original product.
and:
One way scientists demonstrate bioequivalence is to measure the time it takes the generic drug to reach the bloodstream and its concentration in the bloodstream in 24 to 36 healthy, normal volunteers. This gives them the rate and extent of absorption-or bioavailability-of the generic drug, which they then compare to that of the pioneer drug. The generic version must deliver the same amount of active ingredients into a patient's bloodstream in the same amount of time as the pioneer drug.
So, according to the FDA, adequate studies must be provided to show that the generic drug is equivalent to the brand name drug. The above referenced study seems to indicate that the blood levels were higher in the generic group and that side effect rates were higher. The question is who to believe? My own bias is against the pharmaceutical companies as they have an inherent bias in supporting studies that will be to their favor. The authors of the above referenced study have received grants and support from the manufacturer of Effexor (Wyeth). My own impression (and just my opinion) is that Journal of Clinical Psychiatry tends to publish articles that favor the pharmaceutical industry.
Nonetheless, the article has served its purpose in created doubt amongst people that the generic drug may not be equal to the brand name.
After doing a little more research, it is interesting that they did not find differences in bioavailability between citalopram and Celexa. The history of this finding was interesting enough that I'll be creating a separate thread here on the topic soon.