First You don't give Finasteride for prostate cancer. Please, don't be so ignorant.
Second, you don't even use Finasteride to prevent prostate cancer, do a reasearch on pubmed and see how bad that clinical trials went.
DHT is not the enemy . So simple like that, is is a real important sexual hormone in the male body. What is even worst. the 5-alpha reductase enzyme is part of many, many reaction on the body, not just in convert T to DHT. For example, it convert pregnolone into alopregnolone, an neurohormone with anti anxiety effect in the brain.
Of course, Merck never told you that.
What else Merk never told you ? That DHT goes hand to hand with Estrogen, if you lower one, you have to lower the other to don't have side effects. What else ? Progesterone , a really important hormone in MALES too, goes to zero, since progesterone inhibit ( in a feed back way ) aromatasa and the reductasa. So, when the brain sense that there is less DHT, the secretion of Progesterone is reduce it. Etc, etc, etc .
Clearly, yo are not old enough. You will not say,"hey , I am forty, it fucked my best years of sex, but, hey ! I have my hair " hair without good sex, is like being reach and can't spend the money. Simple , useless.