This is a second hand opinion. That said the answer is; YES it will reduce more than you think! Finasteride is an anti-androgen. It will block DHT production, and this is what makes you male. If you are worried about shedding, get into pumpkin seeds and tocotrienol supplements. Palm oil extracts are the best source. And perhaps maybe even topical modoxidil (Rogaine). There are some horror stories about guys losing libido, sexual dysfunction, and even losing muscle strength. And it may not come back after you stop taking it. The effects can persist for months or years. You could be one of the lucky ones and not get any of the side effects. Do you feel lucky? Is it worth the risk?
I looked into this a few years back, and I decided it was not worth it.