Finasteride, sold under the brand names Proscar and Propecia among others, is a medication used mainly to treat an enlarged prostate or hair loss in men.[2] It can also be used to treat excessive hair growth in women and as a part of hormone therapy for transgender women.[3][4] It is taken by mouth.[2] Side effects are generally mild.[5] Some men experience sexual dysfunction, depression, anxiety, or breast enlargement.[6] It may also increase the risk of certain forms of prostate cancer.[6] Finasteride is a 5α-reductase inhibitor, and therefore an antiandrogen.[7] It works by decreasing the production of dihydrotestosterone (DHT) by about 70%, including in the prostate gland and the scalp.[2] Finasteride was patented in 1984 and approved for medical use in 1992.[8] It is available as a generic medication.[9] A month supply in the United Kingdom costs the NHS about 0.89 £ per month as of 2019.[10] In the United States the wholesale cost of this amount is about 2.34 USD.[11] In 2016 it was the 75th most prescribed medication in the United States with more than 10 million prescriptions.[12] Physicians sometimes prescribe finasteride for the treatment of benign prostatic hyperplasia (BPH), informally known as an enlarged prostate. Finasteride may improve the symptoms associated with BPH such as difficulty urinating, getting up during the night to urinate, hesitation at the start and end of urination, and decreased urinary flow. It provides less symptomatic relief than alpha-1 blockers such as tamsulosin and symptomatic relief is slower in onset (six months or more of treatment with finasteride may be required to determine the therapeutic results of treatment). Symptomatic benefits are mainly seen in those with prostate volume > 40 cm3. In long-term studies finasteride but not alpha-1 inhibitors reduce the risk of acute urinary retention (−57% at 4 years) and the need for surgery (−54% at 4 years). If the drug is discontinued, any therapeutic benefits are reversed within about 6–8 months.[13][5][14] A 2010 Cochrane review found a 25 or 26% reduction in the risk of developing prostate cancer with 5α-reductase inhibitor chemoprevention.[15] A followup study of the Medicare claims of participants in a 10-year Prostate Cancer Prevention Trial suggests a significant reduction in prostate cancer risk is maintained even after discontinuation of treatment.[16] However, 5α-reductase inhibitors have been found to increase the risk of developing certain rare but aggressive forms of prostate cancer (27% risk increase), although not all studies have observed this.[17] In addition, there is no negative impact of 5-α-reductase inhibitor on survival rates of people with prostate cancer.[17] Finasteride is also used to treat male pattern baldness (androgenetic alopecia) in men, a condition that develops in up to 80% of Caucasian men.[18][19] In the United States, finasteride and minoxidil are the only two FDA approved drugs for the treatment of male pattern hair loss as of 2017.[20] Treatment with finasteride slows further hair loss[21] and provides about 30% improvement in hair loss after six months of treatment, with effectiveness persisting as long as the drug is taken.[6] Taking finasteride leads to a reduction in scalp and serum DHT levels; by lowering scalp levels of DHT, finasteride can maintain or increase the amount of terminal hairs in the anagen phase by inhibiting and sometimes reversing miniaturization of the hair follicle. Finasteride is most effective on the vertex but can reduce hair loss in all areas of the scalp.[22][23] Finasteride has also been tested for pattern hair loss in women; however, the results were no better than placebo.[24] Finasteride has been found to be effective in the treatment of hirsutism (excessive facial and/or body hair growth) in women. In a study of 89 women with hyperandrogenism due to persistent adrenarche syndrome, finasteride produced a 93% reduction in facial hirsutism and a 73% reduction bodily hirsutism after 2 years of treatment. Other studies using finasteride for hirsutism have also found it to be clearly effective.[3] Finasteride is sometimes used in hormone replacement therapy for transgender women due to its antiandrogenic effects, in combination with a form of estrogen. However, little clinical research of finasteride use for this purpose has been conducted and evidence of safety or efficacy is limited.[4] Moreover, caution has been recommended when prescribing finasteride to transgender women, as finasteride may be associated with side effects such as depression, anxiety, and suicidal ideation, symptoms that are particularly prevalent in the transgender population and in others at high risk already.[25] Finasteride may cause birth defects in a male fetus if a pregnant woman takes finasteride or is exposed to finasteride pill fragments.[26][27] It is classified in the FDA pregnancy category X.[19] Finasteride induces ambiguous genitalia in male fetuses when given to pregnant rhesus monkeys, whereas no abnormalities are observed in female fetuses.[13][19][28] There is a single case report of accidental finasteride exposure during pregnancy, although in a female infant.[29][30] A 2010 Cochrane review concluded that adverse effects from finasteride are rare when used for BPH.[5] When finasteride was originally approved for hair loss in 1997, the FDA reported that it appeared well tolerated, with the most common side effects being related to sexual function.[31] The FDA has added a warning to 5α-reductase inhibitors concerning an increased risk of high-grade prostate cancer, as the treatment of BPH lowers PSA (prostate-specific antigen), which could mask the development of prostate cancer.[32][33] Although overall incidence of male breast cancer in clinical trials for finasteride 5 mg was not increased, there are post-marketing reports of breast cancer in association with its use, though available evidence does not provide clarity as to whether there is a causative relationship between finasteride and these cancers.[19][34] Some men develop gynecomastia (breast development or enlargement) following finasteride usage.[35][36][37][38] The risk of gynecomastia with 5α-reductase inhibitors is low at about 2.8%.[39] Depressive symptoms and suicidality has been reported.[40] Finasteride causes short-term sexual dysfunction in some men.[6] Whether finasteride causes long-term sexual dysfunction in some men after stopping drug treatment is unclear.[41] There are case reports of persistent diminished libido or erectile dysfunction after stopping the drug and the FDA has updated the label to inform people of these reports.[6][42][39] The 2010 Cochrane review found that compared with placebo, men taking finasteride are at increased risk for impotence, erectile dysfunction, decreased libido, and ejaculation disorder for the first year of treatment; the rates of these effects became indistinguishable from placebo after 2–4 years and these side effects usually got better over time.[5] Another 2010 review found that when used for hair loss finasteride increased rates of sexual problems.[43] A 2016 meta-analysis found that sexual dysfunction, including erectile dysfunction, loss of libido, and reduced ejaculate, may occur in 3.4 to 15.8% of men treated with finasteride or dutasteride.[17] This adverse effect has been linked to lower quality of life and can cause stress in relationships.[44] Finasteride has been studied in humans at single doses of up to 400 mg and at continuous dosages of up to 80 mg/day for three months, without adverse effects observed.[19][13][45] These doses are many times in excess of the typical dosages of finasteride of 1 mg/day and 5 mg/day that are used clinically.[19][13] There is no specific recommended antidote for finasteride overdose.[19][13] No significant drug interactions have been observed between finasteride and a limited selection of medications.[46] Finasteride is a 5α-reductase inhibitor.[19][1] It is specifically a selective inhibitor of the type II and III isoforms of the enzyme.[1][47][48] By inhibiting these two isozymes of 5α-reductase, finasteride reduces the formation of the potent androgen dihydrotestosterone (DHT) from its precursor testosterone in certain tissues in the body such as the prostate gland, skin, and hair follicles.[1][49] As such, finasteride is a type of antiandrogen, or more specifically, an androgen synthesis inhibitor.[50][51] However, some authors do not define finasteride as an "antiandrogen," a term which can refer more specifically to antagonists of the androgen receptor.[52] Finasteride results in a decrease of circulating DHT levels by about 65 to 70% with an oral dosage of 5 mg/day and of DHT levels in the prostate gland by up to 80 to 90% with an oral dosage of 1 or 5 mg/day.[47][53][54] In parallel, circulating levels of testosterone increase by approximately 10%, while local concentrations of testosterone in the prostate gland increase by about 7-fold and local testosterone levels in hair follicles increase by around 27 to 53%.[55][56] An oral dosage of finasteride of only 0.2 mg/day has been found to achieve near-maximal suppression of DHT levels (68.6% for 0.2 mg/day relative to 72.2% for 5 mg/day).[56][57] Finasteride does not completely suppress DHT production because it lacks significant inhibitory effects on the 5α-reductase type I isoenzyme, with more than 100-fold less inhibitory potency for type I as compared to type II (IC50 = 313 nM and 11 nM, respectively).[19][1] This is in contrast to inhibitors of all three isoenzymes of 5α-reductase like dutasteride, which can reduce DHT levels in the entire body by more than 99%.[47] In addition to inhibiting 5α-reductase, finasteride has also been found to competitively inhibit 5β-reductase (AKR1D1).[58] However, its affinity for the enzyme is substantially less than for 5α-reductase (an order of magnitude less than for 5α-reductase type I) and hence is unlikely to be of clinical significance.[58] As of 2012, the tissues in which the different isozymes of 5α-reductase are expressed are not fully clear.[49] This is because different investigators have obtained varying results with different reagents, methods, and tissues examined.[49] However, the different isozymes of 5α-reductase appear to be widely expressed, with notable tissues including the prostate gland, seminal vesicles, testes, epididymides, skin, hair follicles, liver, kidneys, and brain, among others.[49] By inhibiting 5α-reductase and thus preventing DHT production, finasteride reduces androgen signaling in tissues like the prostate gland and the scalp. In the prostate, this reduces prostate volume, which improves BPH and reduces risk of prostate cancer. Finasteride reduces prostate volume by 20 to 30% in men with benign prostatic hyperplasia.[59] Inhibition of 5α-reductase also reduces epididymal weight, and decreases motility and normal morphology of spermatozoa in the epididymis.[60] Neurosteroids like 3α-androstanediol (derived from DHT) and allopregnanolone (derived from progesterone) activate the GABAA receptor in the brain; because finasteride prevents the formation of neurosteroids, it functions as a neurosteroidogenesis inhibitor and may contribute to a reduction of GABAA activity. Reduction of GABAA receptor activation by these neurosteroids has been implicated in depression, anxiety, and sexual dysfunction.[61][62][63] The mean oral bioavailability of finasteride is approximately 65%.[1] The absorption of finasteride is not affected by food.[19][13] At steady-state with 1 mg/day finasteride, mean peak concentrations of finasteride were 9.2 ng/mL (25 nmol/L).[19] Conversely, following a single 5 mg dose of finasteride, mean peak levels of finasteride were 37 ng/mL (99 nmol/L), and plasma concentrations increased by 47 to 54% following 2.5 weeks of continued daily administration.[13] The volume of distribution of finasteride is 76 L/kg.[1] Its plasma protein binding is 90%.[1] The drug has been found to cross the blood–brain barrier, whereas levels in semen were found to be undetectable.[1] Finasteride is extensively metabolized in the liver, first by hydroxylation via CYP3A4 and then by aldehyde dehydrogenase.[1] It has two major metabolites, which are the tert-butyl side chain monohydroxylated and monocarboxylic acid metabolites.[1] These metabolites show approximately 20% of the inhibitory activity of finasteride on 5α-reductase.[1] Hence, the metabolites of finasteride are not particularly active.[1] The drug has a terminal half-life of 5 to 6 hours in adult men (18–60 years of age) and a terminal half-life of 8 hours or more in elderly men (more than 70 years of age).[1] It is eliminated as its metabolites 57% in the feces and 40% in the urine.[1] Finasteride, also known as 17β-(N-tert-butylcarbamoyl)-4-aza-5α-androst-1-en-3-one, is a synthetic androstane steroid and 4-azasteroid.[46][64] It is an analogue of androgen steroid hormones like testosterone and DHT.[46] As an unconjugated steroid, finasteride is a highly lipophilic compound.[46][65] In 1942, James Hamilton observed that prepubertal castration prevents the later development of male pattern baldness in mature men.[66] In 1974, Julianne Imperato-McGinley of Cornell Medical College in New York attended a conference on birth defects. She reported on a group of intersex children in the Caribbean who appeared sexually ambiguous at birth, and were initially raised as girls, but then grew external male genitalia and other masculine characteristic after onset of puberty. These children, despite being raised as girls until puberty, were generally heterosexual, and were termed "Guevedoces" by their local community, which means "penis at twelve" in Spanish.[67] Her research group found these children shared a genetic mutation, causing deficiency of the 5α-reductase enzyme and male hormone dihydrotestosterone (DHT), which was found to have been the etiology behind abnormalities in male sexual development. Upon maturation, these individuals were observed to have smaller prostates which were underdeveloped, and were also observed to lack incidence of male pattern baldness.[68][69] In 1975, copies of Imperato-McGinley's presentation were seen by P. Roy Vagelos, who was then serving as Merck's basic-research chief. He was intrigued by the notion that decreased levels of DHT led to the development of smaller prostates. Dr. Vagelos then sought to create a drug which could mimic the condition found in these children to treat older men who were suffering from benign prostatic hyperplasia.[70] Finasteride was developed by Merck under the code name MK-906.[46] In 1992, finasteride (5 mg) was approved by the U.S. Food and Drug Administration (FDA) for treatment of BPH, which Merck marketed under the brand name Proscar. In 1997, Merck was successful in obtaining FDA approval for a second indication of finasteride (1 mg) for treatment of male pattern hair loss, which was marketed under the brand name Propecia.[71] It was the first 5α-reductase inhibitor to be introduced and was followed by dutasteride in 2001.[72] The first study of finasteride in the treatment of hirsutism in women was published in 1994.[73] Finasteride is the generic name of the drug and its INN, USAN, BAN, and JAN, while finastéride is its DCF.[74][75][76][77] It is also known by its former developmental code names MK-906, YM-152, and L-652,931.[74][75][76][77] Finasteride is marketed primarily under the brand names Propecia, for pattern hair loss, and Proscar, for BPH, both of which are products of Merck & Co.[77] There is 1 mg of finasteride in Propecia and 5 mg in Proscar. Merck's patent on finasteride for the treatment of BPH expired in June 2006.[78] Merck was awarded a separate patent for the use of finasteride to treat pattern hair loss and it expired in November 2013.[79] Finasteride is also marketed under a variety of other brand names throughout the world.[77] Men in the U.S. 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".[80] In 2016, Merck was a defendant in approximately 1,370 product liability lawsuits which had been filed by customers alleging they have experienced persistent sexual side effects following cessation of treatment with finasteride.[81] From 2005 to 2009, the World Anti-Doping Agency banned finasteride because it was discovered that the drug could be used to mask steroid abuse.[82] It was removed from the list effective January 1, 2009, after improvements in testing methods made the ban unnecessary.[83] Athletes who used finasteride and were banned from international competition include skeleton racer Zach Lund, bobsledder Sebastien Gattuso, footballer Romário, and ice hockey goaltender José Théodore.[83][84] The Food and Drug Administration advises that donation of blood or plasma be deferred for at least one month after taking the last dose of finasteride.[85] Harold Bornstein, the former[86] personal physician of United States President Donald Trump, stated in 2017 that Trump was taking finasteride to promote hair growth.[87] DHT may be involved in the cause of acne, and 5α-reductase inhibitors might be effective in the treatment of the condition.[88][89] A small retrospective study reported that finasteride was effective in the treatment of acne in women with normal testosterone levels.[90][89] A randomized controlled trial found that finasteride was less effective than flutamide or an ethinylestradiol/cyproterone acetate birth control pill in the treatment of acne in women with high androgen levels.[90] Androgens and estrogens may be involved in the cause of hidradenitis suppurativa (acne inversa).[91][92] Two case series have reported that finasteride is effective in the treatment of hidradenitis suppurativa in girls and women.[90]