How Do Doctors Know That Finasteride Only Affects the Hairs that Were Destined to Fall Out?

Recently I have noticed thinning at the front of my hairline. I would like to start using propecia but I have heard horror stories from my friends. I was told by one that it actually increased his balding. Apparently he started shedding more with propecia, and what started as a receding hairline also started affecting him in the crown. Is this possible? Can propecia actually speed up balding, or were those just hairs that were falling out anyways? I also heard that once off propecia, the rate of balding increases. I understand “catch up” time, but how do doctors actually know that propecia just affects hair that was destined to fall out? Thank you for your time! Hope to hear from you soon!

When a person who is not balding takes Propecia, nothing happens to the hair. That clearly shows that Propecia does not impact normal hair.