Treatment of young subjects is of increasing concern due to accumulating evidence that daily use of oral finasteride has several severe adverse effects… Since finasteride inhibits T conversion into DHT, which is responsible for most androgen activity, it is plausible that prolonged finasteride use in predisposed individuals could simulate the effects of aging in young men. Since some of the effects of androgen inhibition cannot be reversed once local androgen levels are re-established, it is temping to speculate that patients could still suffer from adverse sexual effects several months or even permanently after finasteride discontinuation.
Men in the US and Canada concerned about persistent sexual side effects "coined the phrase 'post finasteride syndrome', which they say is characterized by sexual, neurological, hormonal and psychological side effects that can persist in men who have taken finasteride for hair loss or an enlarged prostate".   In 2012, a health advocacy group called the Post-Finasteride Syndrome Foundation was formed with the primary goal of finding a cure for the reported syndrome and a secondary goal of raising awareness.  According to the company's 1Q2016 financial filing, Merck is a defendant in 1,385 product liability lawsuits which have been filed by customers alleging they have experienced persistent sexual side effects following cessation of treatment with finasteride.