Much like its cousin minoxidil, finasteride didn’t start out as a hair loss treatment.
Doctors originally used it to treat benign prostatic hyperplasia (BPH) or enlarged prostate. Most of the symptoms of BPH affected the urination process because of the pressure applied by the prostate to the base of the bladder and the urethra. To keep it simple, finasteride inhibits the production of dihydrotestosterone (DHT) which produces the enzyme that increases prostate volume.
DHT is also an agonist for androgen receptor activation, which is one of the main contributors to androgenetic alopecia, better known as male pattern baldness. By decreasing the androgen in the body, hair follicles on the scalp maintain their original levels of production.
In 1997, Merck, who originally developed the BPH drug called Proscar, obtained FDA approval to market Propecia, aimed at combating hair loss.
Adverse effects are rare. There are however cases of high-grade prostate cancer related to usage, as well as masking the development of prostate cancer. Other more common side effects of finasteride include increased risks of ejaculation disorder, erectile dysfunction, impotence, and decreased sex drive. Some experience a persistence of these disorders, while many more report an immediate improvement upon cessation of use.
Merck’s patent ran out in November of 2013, so if you’re in the market, be on the lookout for generics.