why does propecia take so long to work?

[mothernature]

can anyone tell me the pharmacological mechanism behind the use of propecia? I've read that it lowers DHT by up to 60-80% in the body within the first week of taking it, but why is it then that it can take a minimum of 3 months and up to a year to even stop hair falling out? If it attacks the enzyme which is the root cause of the falling out then why does its effect take months and months to show?

Why am I shedding hair all of a sudden? I think I started shedding before taking prop (been 1 and a half months now) and it still hasn't stopped..