Studies must be concluded prior to releasing a drug to the market.
This kind of studies are not usually accurate because the number of persons being tested is not great enough.
Only after prescribing the drug to the large masses for a considerable time the real statistics can be put together.
It's possible that drugs like Finasteride (and others) help only so little or not at all, but this can be concluded only in time.
In case of Propecia (Finasteride) the side effects list is being updated since 2009.
Shaving gel and a razor bring up less side effects.