RESEARCH MONOGRAPH · KDC-MN-279

Finasteride

May 9, 2026 Kodiac biolabs Research Revised May 30, 2026 3 min read

Our opinion on this compound

81/100

Type-II 5-AR inhibitor with real efficacy and a post-finasteride syndrome literature thats more complicated than either camp admits

Finasteride is one of those compounds where the clinical story is more complicated than either the marketing or the critical literature wants to admit. Merck's 4-azasteroid 5-alpha-reductase inhibitor, with preferential affinity for the type-II isoform, was developed in the 80s for BPH (Proscar at 5 mg) and later approved for androgenetic alopecia (Propecia at 1 mg). The DHT-reduction mechanism is unambiguous and the clinical efficacy in both indications is well-established.

The mechanism is well-mapped. Type-II 5-AR is the dominant isoform in prostate, hair follicles, and external genitalia; type-I is more important in skin generally and liver. Finasteride is a relatively selective type-II inhibitor at the doses used clinically, producing roughly 65-70% reduction in serum DHT while testosterone and other androgens remain relatively unchanged. The prostate-specific volume reduction and the hair-density improvements in androgenetic alopecia are direct consequences of local DHT suppression.

What we like is the clinical data quality. The PLESS trial established BPH efficacy with hard endpoints (urinary symptom score, prostate volume, surgical intervention rates over 4 years); the long-term hair-loss data is reasonable, and the PCPT trial unexpectedly showed prostate cancer risk reduction (with a separate signal about Gleason score that has been extensively debated). Effect sizes are real and reproducible.

What we dont like is how polarized the post-finasteride syndrome discussion has become. The clinical literature on persistent sexual, mood, and cognitive symptoms after finasteride discontinuation is real but methodologically difficult, with case series and observational data that's hard to fully evaluate against placebo-controlled trial data. Our honest view is that PFS is a real signal in some subset of users with currently poorly characterized incidence, mechanism, and predictors. Neither dismissal nor catastrophization fits the data well. The work by Khera and others on steroidogenic enzyme alterations in CNS tissue is some of the more interesting mechanistic exploration of why persistent symptoms might occur.

Sourcing is excellent. Off-patent globally, multiple generic manufacturers, pharmacopoeial standards. Research-grade material is straightforward.

Workhorse with the underappreciated angle being the unresolved questions about long-tail neurological effects. Mechanism is textbook, reliability is high for primary indications, and the PFS literature is one of the harder things to think clearly about in this space. Honestly worth more research attention than it gets.

Evidence: 88
Mechanism: 88
Reliability: 85
Safety: 60
Sourcing: 92
Value: 85
Signal: 75
Staying power: 75

Plain-language summary Intrigue 65 / 100

Finasteride (Proscar at 5 mg, Propecia at 1 mg) is a 1990s drug that blocks the type II 5-alpha-reductase enzyme, which converts testosterone into the more potent dihydrotestosterone (DHT) in the prostate, scalp, and skin. It cuts plasma DHT by about 65 to 70 percent at the 5 mg dose. Approved for benign prostate enlargement and male-pattern hair loss, with both indications driven by reducing local DHT. The hair-loss application is the famous one. Side effects include sexual dysfunction in a minority of users, with persistent symptoms after discontinuation (post-finasteride syndrome) reported in case series, though the frequency and biological basis remain debated. Not stocked by Kodiac. This monograph is provided for research and educational reference.

Intrigue 0–100 blends mechanism novelty, evidence strength, and translational potential. Kodiac editorial, not peer-reviewed.

5-alpha-reductase type II inhibitor

A 4-azasteroid 5-alpha-reductase type II inhibitor; approved for benign prostatic hyperplasia and androgenetic alopecia.

Abstract

Finasteride (N-(2-methyl-2-propanyl)-3-oxo-(5-alpha,17-beta)-4-azaandrost-1-ene-17-carboxamide; CAS 98319-26-7; molecular formula C23H36N2O2; molecular weight 372.55) is a 4-azasteroid 5-alpha-reductase inhibitor developed at Merck and approved by the FDA in 1992 (Proscar 5 mg for BPH) and 1997 (Propecia 1 mg for androgenetic alopecia). Mechanism: selective inhibition of 5-alpha-reductase type II isozyme (Ki approximately 10 nM), which converts testosterone to dihydrotestosterone (DHT) in prostate, hair follicle, and skin. Plasma DHT is reduced by approximately 65 to 70 percent at the 5 mg dose. Type I 5AR (in liver, skin, and brain) is largely unaffected. Plasma half-life is approximately 6 to 8 hours; metabolism is via CYP3A4. The compound has well-documented sexual side effects (reduced libido, erectile dysfunction in approximately 1 to 5 percent of users) and the controversial post-finasteride syndrome (persistent symptoms after discontinuation, debated as a clinical entity). Used as the reference 5AR-II selective inhibitor in mechanism studies and as a positive control in androgen pathway research.

Mechanism of action

Selective 5-alpha-reductase type II inhibition; reduces conversion of testosterone to DHT. Largely spares type I 5AR.

Reported research dose ranges

Clinical 1 mg (alopecia) or 5 mg (BPH) in the published literature. Rodent studies 0.5 to 5 mg/kg.

References

Steiner JF. Clinical pharmacokinetics and pharmacodynamics of finasteride. Clin Pharmacokinet 1996.
Kaufman KD, et al. Finasteride in the treatment of men with androgenetic alopecia. J Am Acad Dermatol 1998.
Trost L, et al. The recently published Cochrane review of post-finasteride syndrome. Sex Med Rev 2018.

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KDC-MN-279

The full reference document is provided strictly for research use only. It reports research dose ranges from the published literature, not instructions for use in humans or animals.

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Finasteride

Abstract

Mechanism of action

Reported research dose ranges

References

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Abstract

Mechanism of action

Reported research dose ranges

References

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