Even if you were the best responder to finasteride in human history, it'd be impossible to tell at the three week mark! It's much too early. Even three months is far too early.
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My timeline:
December 2011: Start Nizoral (it's cheap, didn't need a prescription, no-brainer really) Use every 5 to 7 days, depending in various factors.
8th January 2012: get my Propecia prescription at last. Start taking immediately.
January/February: Shedding phase. HUGE shed, very visible, pillows, bathroom, under computer keyboard everywhere.
Later February: No shedding of hairs. Scalp itch completely gone.
30th July: Still on Propecia and using Nizoral, no shedding at all, but NO regrowth of initial shed either.
The KEY thing is that I HAVEN'T shed hairs at all since the initial shed. So why is it taking so long to regain my initial loss? I'd look halfway-decent with that restored, let alone any other bonus regrowth.