Generics must be proven (to the FDA) to be as safe as the original drug and the total absorption % is about the same, but they don't have to have the same RATE of absorption. For example, the original brand may take 15 minutes to absorb into the blood, but a generic may take 30 minutes to completely absorb. So brand name Tylenol make relieve your headache in 15 minutes, but some generic Tylenol may take 30-45 minutes.
As a rule of thumb, most people don't take chances on generics when it's something as important as treating strokes, heart attacks, blood clots, dysthymia, thyroid, etc. If you have a stroke or heart attack, you don't want to wait 45 minutes for a drug to work.