How finasteride works
First, a note on terminology: androgens are hormones that regulate the development and maintenance of male characteristics. Finasteride inhibits an enzyme that converts the androgen testosterone to a more potent androgen called dihydrotestosterone, or DHT. DHT is understood to degrade hair follicles in the scalp, contributing to male pattern hair loss. By reducing available DHT, finasteride is supposed to slow or stop hair loss. For more details, see PubChem and section 12 of Propecia Prescribing Information.
[Editorial comment: You can read about my experience with finasteride here. Research has suggested that adverse effects of the drug were not understood at the time of approval in 1997 (see Belknap meta-analysis in JAMA Dermatology). Today there are significant unresolved concerns about potential harms of the drug when used for hair loss in young men (see Research by theme). The safety profile of this drug as a treatment for hair loss is questionable at best.]
Official drug label
Advice from peers
If you are considering taking finasteride, see peer advice on finasteride. This reflects the perspective of men who have taken finasteride and gotten adverse effects, incorporating other sources including adverse event reports, social media activity and published research.
Long-term syndrome and persistent adverse effects
For a description of a constellation of symptoms that can persist long after stopping finasteride, see Long-term effects: a ‘post-finasteride syndrome’.
In their own words is a selection of reports by men who experienced lasting symptoms after stopping finasteride.