In the first instance, I am aware of the side effect profile noted with finasteride.  I've also read the clinical studies describing its utility in the setting of pattern hair loss.  As you may be aware finasteride is a specific inhibitor of the type two 5 alpha-reductase enzyme, the enzyme implicated in the conversion of testosterone to the pathologic metabolite 5 alpha-dihydrotestosterone.  However, because both 5 alpha-reductase (5AR) isoforms (type 1 and type 2) may play a role in patten hair loss, the activity of type 1 5AR is still operative even in the face of therapeutic challenge with finasteride.