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5α-Reductase Inhibitors (5-ARIs)

Clinical role​

Finasteride and dutasteride reduce conversion of testosterone to dihydrotestosterone (DHT). In men with symptomatic BPH and an enlarged prostate, they gradually reduce gland volume and lower the risk of acute retention and BPH surgery. They are a long-term option; symptom benefit usually takes 3–6 months, so set that expectation at the start. AUA 2026 calls for reassessment within six months of starting treatment, with a standardized symptom score and adverse-event review .[23] EAU 2026 advises review at 12 weeks and six months for 5-ARI monotherapy, or at four to six weeks for alpha-blocker/5-ARI combination.[24] They do not provide immediate relief of retention.[1][2] The Proscar label advises careful monitoring for obstructive uropathy in patients with large residual urinary volume, severely diminished urinary flow, or both; such patients may not be candidates for finasteride.[1]

EAU 2026 strongly recommends 5-ARIs for moderate-to-severe LUTS with increased progression risk, using prostate volume >40 mL as an example, and strongly recommends counseling about the slow onset and the side effects.[3] After 2 to 4 years, EAU reports IPSS improvement of about 15–30%, prostate volume reduction of 18–28% and Qmax gain of 1.5–2.0 mL/s (level 1b).[3] It notes that finasteride may not beat placebo in glands under 40 mL, whereas dutasteride has shown benefit at 30–40 mL.[3] AUA 2026 recommends offering a 5-ARI for symptoms when estimated prostate volume is >30 cc and/or PSA >1.5 ng/mL ; reducing progression in this enlarged-prostate group is a Clinical Principle.[4] The alpha-blocker/5-ARI combination uses the same enlargement criteria .[4] PSA must first be interpreted in its clinical context.[3][4]

Mechanism and expected effects​

5α-reductase converts testosterone to dihydrotestosterone (DHT), which is more potent at the androgen receptor and drives prostatic growth. Blocking it reduces growth signaling, increasing apoptosis and atrophy of prostatic tissue. Two isoenzymes matter: type 2 predominates in prostate tissue, while type 1 is found in skin, scalp and liver. Finasteride inhibits type 2 only; dutasteride inhibits both.[1][2][4]

ParameterFinasteride 5 mgDutasteride 0.5 mg
Serum DHT suppressionAbout 70%[1]About 93–95% (median, years 1–4)[2]
Prostate volume−17.9% at 4 years (PLESS MRI subset remaining on therapy) vs +14.1% with placebo[1]−24.7% at 12 months and −26.7% at 24 months vs −3.4% and −2.2% with placebo[2]
Symptom scoreModest gain over placebo (see PLESS below)AUA-SI change at 24 months −3.8 vs −1.6 with placebo in men ≥40 cc (difference −2.2)[2]
Serum testosteroneMedian rise about 10–20%, within the physiologic range[1]Median rise about 19–26% over years 1–4, within the physiologic range[2]

Volume reduction is greater in larger glands. After 14 days of finasteride in healthy male volunteers, DHT returned to pretreatment levels about two weeks after stopping; in patients treated for three months, prostate volume had fallen about 20% and returned close to baseline about three months after stopping.[1] Prostate volume may rebound after 5-ARI withdrawal, while symptom response varies when another LUTS drug continues.[1][4][1][2][4] The deeper DHT suppression with dutasteride has not translated into differences in volume reduction, symptom score or flow in head-to-head data (see EPICS below).

Agents and administration​

AgentAdult BPH regimenImportant formulation and pharmacology points
Finasteride (Proscar)5 mg orally once daily, with or without foodPrimarily type 2 inhibition. The 1-mg alopecia product is a separate indication/dose; in an alopecia trial, median PSA fell 40% at ages 40–49 and 50% at ages 50–60 after 48 weeks. No renal dose adjustment; use caution with liver dysfunction.[1][26]
Dutasteride (Avodart)0.5 mg orally once daily, with or without foodType 1 and 2 inhibition. Swallow capsules whole; do not open or chew. No renal adjustment. Hepatic impairment has not been studied adequately; exposure may increase. The approximately five-week terminal half-life means effects/exposure do not disappear immediately after stopping.[2]
Dutasteride/tamsulosin (Jalyn)0.5/0.4 mg once daily, about 30 minutes after the same mealSwallow whole; the tamsulosin component adds orthostasis, interactions, ejaculation effects and IFIS precautions. See the alpha-blocker hub.[5]

Dosing and organ-function information are from the US labels.[1][2][5] Dutasteride lowers circulating DHT more than finasteride. In EPICS, 1,630 men randomized to the two agents had similar prostate-volume, symptom and flow improvements over 12 months; the primary endpoint was prostate volume, not long-term retention or surgery.[6]

Finasteride has no established clinically important pharmacokinetic interactions in its label. Use caution with chronic potent CYP3A4 inhibition and dutasteride; combination products require review of both ingredients. Adding an alpha-blocker can increase adverse effects even when there is no major pharmacokinetic interaction.[1][2][5]

What the major BPH trials establish​

EvidencePopulation and interpretation
PLESS3,040 men with enlarged prostates randomized to finasteride 5 mg or placebo for four years. Finasteride reduced BPH surgery by 55% (5% vs 10%) and acute urinary retention by 57% (3% vs 7%), with a modest symptom-score gain among men who completed the study (mean decrease 3.3 vs 1.3).[20]
MTOPS3,047 men, mean follow-up 4.5 years. Doxazosin, finasteride and combination reduced composite clinical progression by 39%, 34% and 66%, respectively, versus placebo. The composite was largely symptom worsening; only finasteride and combination reduced retention and invasive therapy.[7] In a secondary analysis by baseline transrectal-ultrasound prostate volume, combination therapy was no better than doxazosin alone for progression, invasive therapy, symptom score or flow when the gland was under 25 mL, but was superior to either drug alone at 25 to under 40 mL and at 40 mL or more.[22]
CombAT4,844 men with IPSS ≥12, prostate volume ≥30 mL and PSA 1.5–10 ng/mL; four years, no placebo arm. Dutasteride/tamsulosin improved symptoms and reduced overall clinical progression more than either alone. For the primary AUR-or-surgery endpoint, combination was superior to tamsulosin but not significantly superior to dutasteride.[8]

Combination therapy can be considered initially in a symptomatic patient with substantial progression risk. Typical candidates have moderate-to-severe LUTS with a gland >30 cc and/or PSA >1.5 ng/mL; this includes men whose symptoms persist on alpha-blocker monotherapy and who meet the enlargement criterion.[4] The alpha-blocker supplies early relief while the 5-ARI effect builds over months. In CombAT, prostate biopsies obtained for PSA changes did not show a higher rate of high-grade cancer with combination therapy, and in a trial of immediate dutasteride and tamsulosin versus watchful waiting with delayed tamsulosin, withdrawals for erectile and ejaculatory adverse events were higher in the combination group.[4]

Stopping one component after a period of control is not well studied. In Lee’s RCT of men whose IPSS and prostate volume improved on combination therapy, IPSS deterioration of at least 2 points at 24 months occurred in 20/72 (27.8%) after alpha-blocker withdrawal and 19/72 (26.4%) after 5-ARI withdrawal.[27] In a separate RCT with 12 months after withdrawal, men with larger prostates more often resumed the 5-ARI, and TURP was more frequent after 5-ARI than alpha-blocker withdrawal (14.5% vs 7.1%).[28] Daily tadalafil 5 mg plus a 5-ARI is a conditional AUA option for a gland >30 cc and/or PSA >1.5 ng/mL, with or without ED.[4] For tadalafil with finasteride to initiate BPH treatment, the US CIALIS label recommends use for up to 26 weeks because the incremental benefit of tadalafil beyond 26 weeks is unknown.[25] Balance expected benefit against sexual effects, treatment burden and the time needed for the 5-ARI to work.[3][4]

PSA interpretation: use the drug-specific timeline and the trend​

5-ARIs commonly lower PSA by about half, including when prostate cancer is present. Record the drug, start date and adherence wherever PSA is interpreted.[1][2]

Product labelSerial measurementsComparing one value with untreated reference ranges
Finasteride 5 mgEstablish a new baseline after at least six months; monitor periodicallyAfter ≥6 months, double the observed value for comparison[1]
Dutasteride 0.5 mgEstablish a new baseline after at least three months; monitor periodicallyAfter ≥3 months, double the observed value for comparison[2]

Doubling is an approximate interpretation adjustment, not the patient's exact untreated or “true” PSA. Evaluate a confirmed increase from the on-treatment nadir, even if the number falls within an untreated normal range. Review adherence and other explanations. Percent free PSA generally does not require the same adjustment.[1][2]

The 2019 Veterans Affairs cohort of 80,875 men with prostate cancer associated prediagnostic 5-ARI use with longer diagnostic delays and worse cancer-specific outcomes. This observational finding reinforces the need to recognize PSA suppression.[9]

Sexual effects, fertility and breast symptoms​

Discuss erectile dysfunction, reduced libido, lower ejaculate volume and breast tenderness or enlargement before treatment; AUA 2026 recommends counseling on side effects and on the effect on prostate cancer diagnosis when prescribing .[4] Frequency varies by product, dose, follow-up and whether an alpha-blocker is also used. The label rates below are drug-related events against placebo.

Adverse eventFinasteride 5 mg, year 1 (vs placebo)Finasteride, years 2–4 (vs placebo)Dutasteride, months 0–6 (vs placebo)
Erectile dysfunction8.1% (3.7%)5.1% (5.1%)[1]4.7% (1.7%)[2]
Decreased libido6.4% (3.4%)2.6% (2.6%)[1]3.0% (1.4%)[2]
Ejaculatory disorder or reduced volume3.7% volume (0.8%); disorder 0.8% (0.1%)1.5% volume (0.5%)[1]1.4% (0.5%)[2]
Breast enlargement or disorder0.5% (0.1%)1.8% (1.1%)[1]0.5% (0.2%)[2]

Finasteride figures are from the 4-year placebo-controlled study in the Proscar label; dutasteride figures are from the pooled 24-month trials in the Avodart label. New reports of sexual adverse events decline over time, while breast events remain above placebo in later treatment intervals.[1][2] In the REDUCE trial gynecomastia was 1.9% with dutasteride versus 1.0% with placebo[14], and combination with an alpha-blocker adds ejaculatory and erectile effects (MTOPS, CombAT)[1][2]. Most side effects of BPH pharmacotherapy resolve after discontinuation, though some long-term effects are reported and merit shared decision-making.[1][2][4]

Both labels acknowledge reports of sexual problems persisting after discontinuation, with frequency and the independent drug contribution difficult to determine.[1][2] AUA 2026 describes post-finasteride syndrome as a poorly defined constellation, drawn mainly from patient reports in men taking finasteride for hair loss and not substantiated as a clinical entity by RCT adverse-event reporting, but notes that the FDA has amended the labels to warn of it.[4]

The dutasteride label describes a small healthy-volunteer study (27 treated, 23 placebo) with reduced total sperm count, volume and motility; mean sperm count had not returned to baseline at 24 weeks after treatment. In a 52-week study of 27 healthy men taking dutasteride, 2/27 had >90% sperm-count declines from baseline, with partial recovery at the 24-week follow-up after discontinuation; the effect on individual fertility is unknown.[2] Finasteride has postmarketing infertility and poor-semen-quality reports, including improvement after withdrawal. Consider a medication and semen assessment when conception is difficult.[1][2]

In the four-year CombAT trial of men with BPH, composite cardiac failure was reported in 12/1,610 (0.7%) on dutasteride/tamsulosin, 2/1,623 (0.1%) on dutasteride, and 9/1,611 (0.6%) on tamsulosin; the Avodart label says the imbalance’s clinical significance is unknown and a causal relationship has not been established.[2]

Evaluate new breast lumps, nipple discharge or other concerning changes. Male breast cancer has been reported after marketing; a causal relationship is not established.[1][2]

Mood and suicidality: current regulatory distinctions​

The US Proscar label lists depression and suicidal ideation in postmarketing experience.[1] The US Avodart label lists depressed mood.[2]

EMA's 2025 review confirmed suicidal ideation as an adverse effect of oral finasteride 1 mg and 5 mg, with unknown frequency and most reports arising from alopecia use. It found no direct link for dutasteride and recommended a precautionary warning because of the shared mechanism.[10]

The May 11, 2026 MHRA update reinforces baseline mood-history assessment and regular review for psychiatric and sexual effects. Its instructions distinguish stopping finasteride 1 mg and promptly contacting the prescriber if depression or suicidal thoughts develop from seeking prompt medical advice for finasteride 5 mg or dutasteride. Immediate danger or self-harm requires emergency care regardless of drug or dose.[11]

Large observational cohorts have associated 5-ARI use with depression; suicide estimates have been less consistent. Welk's 2017 study was a matched cohort, not a case-control study. The Swedish cohort's early dementia association diminished with longer exposure, raising detection or confounding concerns. Neither observational association nor a proposed neurosteroid mechanism proves a specific cause in an individual patient.[12][13]

Prostate cancer: prevention trials do not remove the label warning​

PCPT (finasteride, 18,882 men, seven years) and REDUCE (dutasteride, 8,231 men with a recent negative biopsy and PSA 2.5–10 ng/mL at ages 50–60 or 3.0–10 ng/mL above age 60, four years) reduced biopsy-detected prostate cancer overall, mainly lower-grade disease: a 24.8% relative reduction in prostate cancer prevalence among men with final-analysis data in PCPT (803/4,368 [18.4%] versus 1,147/4,692 [24.4%]) and a 22.8% stratified relative risk reduction in REDUCE among the 6,729 men who underwent biopsy or prostate surgery (659/3,305 versus 858/3,424).[14][21] Both designs relied on protocol biopsies, and finasteride's PCPT Gleason 7–10 tumors were more frequent (6.4% versus 5.1% of men with final-analysis data).[21] Neither drug is FDA-approved for cancer prevention. Current US labels retain warnings concerning Gleason 8–10 cancers: finasteride 1.8% versus placebo 1.1% in PCPT, and dutasteride 1.0% versus 0.5% in REDUCE.[1][2]

In REDUCE, overall Gleason 7–10 counts were similar (220 versus 233 tumors), while a Gleason 8–10 imbalance emerged in years 3–4 (12 versus 1 tumors).[14] Smaller prostate volume and improved detection can contribute to the findings.[14]

Long-term evidence is reassuring about mortality, with uncertainty:

  • At median 18.4-year follow-up in PCPT, there were 42 prostate-cancer deaths among 9,423 men randomized to finasteride and 56 among 9,457 men randomized to placebo, HR 0.75 (95% CI 0.50–1.12).[15]
  • A 2023 meta-analysis of 11 studies and more than 3.2 million men found no statistically significant mortality association, adjusted HR 1.04 (95% CI 0.80–1.35).[16]

Other uses and perioperative decisions​

  • Prostatic hematuria: after excluding other causes, a 5-ARI may help refractory bleeding attributed to an enlarged prostate. This AUA 2026 Expert Opinion option does not replace hematuria evaluation or acute clot-retention management; do not impose the BPH symptom-response timeline on bleeding control.[4]
  • CP/CPPS: AUA 2025 permits 5-ARIs when there are voiding symptoms from BPH or an enlarged prostate identified by imaging or PSA .[17]
  • TURP bleeding: 5-ARI pretreatment lowers prostatic microvascular density and has reduced perioperative blood loss at TURP in RCTs.[4] A 2026 meta-analysis of 30 randomized TURP trials (2,974 men with BPH) found 82.6 mL less intraoperative blood loss in 11 trials (n = 1,200) and fewer transfusions in 14 trials (n = 1,181) versus placebo or no pretreatment; a subgroup treated for up to two weeks had 57.3 mL less intraoperative blood loss than controls.[30]
  • Active surveillance for prostate cancer: AUA 2026 advises counseling that 5-ARI treatment is unlikely to increase cancer progression . Expect PSA suppression and interpret the trend accordingly.[4]
  • Surgery: UKCPA advises continuing finasteride or dutasteride perioperatively. The old instruction to stop two weeks before prostate surgery to improve bleeding control was unsupported. After effective TURP and successful catheter removal, reassess whether the medication is still needed; no universal 2–4-week restart rule applies.[18][19]

Contraindications and interactions​

Both drugs are contraindicated in pregnancy (see below) and in men with hypersensitivity to the drug; the dutasteride label extends this to other 5-ARIs, and lists serious skin reactions and angioedema among reported reactions. Consider other urologic causes before starting, since BPH and prostate cancer can coexist. Dutasteride is metabolized by CYP3A4/5: use caution with potent chronic inhibitors such as ritonavir. Verapamil and diltiazem raise dutasteride exposure, but no dose change is advised. In a single-sequence crossover study in healthy volunteers, coadministration with dutasteride did not alter the steady-state pharmacokinetics of tamsulosin or terazosin; the effect of either alpha-blocker on dutasteride pharmacokinetics was not evaluated.[1][2]

Pregnancy, handling and donation​

Pregnancy exposure can disrupt development of male external genitalia. These BPH products are not indicated for women or children. A person who is or may be pregnant should not handle crushed or broken finasteride tablets or any dutasteride capsules. If skin contacts a crushed or broken finasteride tablet or a leaking dutasteride capsule, wash immediately with soap and water. Intact finasteride coating prevents contact during ordinary handling. Current narrative pregnancy labeling replaces the old letter-category shorthand.[1][2]

Very small quantities occur in semen; do not turn this into a universal US-label condom requirement for both products. Discuss pregnancy exposure using the applicable product and jurisdiction's labeling. No blood donation during dutasteride treatment or for at least six months after the last dose; check the blood service's deferral rules for other products.[1][2]

Practical points​

  1. Chart the 5-ARI and start date so every clinician interpreting PSA applies the adjustment. Any confirmed rise above the on-treatment nadir needs evaluation.
  2. Set the 3–6-month expectation at the start.
  3. For a large gland with substantial progression risk, consider combination therapy from the outset instead of sequencing. 4.[6] EPICS found similar prostate-volume, symptom and flow changes at 12 months but did not compare long-term retention or surgery.[6]
  4. Screen mood history and counsel on sexual and mood effects before starting; ask about breast symptoms at follow-up.
  5. Reinforce handling precautions for pregnant household members.

See Also​

References​

1. PROSCAR (finasteride 5 mg). US prescribing information. Source.

2. AVODART (dutasteride). US prescribing information. Source.

3. European Association of Urology. Non-neurogenic Male LUTS Guideline, 2026. 5-ARI and combination-treatment sections. Source.

4. Goueli R, et al. Management of LUTS attributed to BPH: AUA Guideline (2026), Part II—Medical Management. Source.

5. JALYN (dutasteride/tamsulosin). US prescribing information. Source.

6. Nickel JC, et al. Comparison of dutasteride and finasteride for treating BPH: EPICS. BJU Int. 2011. Source.

7. McConnell JD, Roehrborn CG, Bautista OM, et al. "The Long-Term Effect of Doxazosin, Finasteride, and Combination Therapy on the Clinical Progression of Benign Prostatic Hyperplasia." N Engl J Med. 2003;349(25):2387–2398. doi:10.1056/NEJMoa030656

8. Roehrborn CG, Siami P, Barkin J, et al. "The Effects of Combination Therapy with Dutasteride and Tamsulosin on Clinical Outcomes in Men with Symptomatic Benign Prostatic Hyperplasia: 4-Year Results from the CombAT Study." Eur Urol. 2010;57(1):123–131. doi:10.1016/j.eururo.2009.09.035

9. Sarkar RR, et al. Association of treatment with 5-ARIs with time to diagnosis and mortality in prostate cancer. JAMA Intern Med. 2019. Source.

10. European Medicines Agency. Finasteride/dutasteride referral and measures to minimize suicidal thoughts, 2025. Source.

11. MHRA. Finasteride and dutasteride: updated safety warnings for psychiatric side effects and sexual dysfunction. May 11, 2026. Source.

12. Welk B, et al. Association of suicidality and depression with 5-ARIs. JAMA Intern Med. 2017. Source.

13. Garcia-Argibay M, et al. Association of 5-ARIs with dementia, depression and suicide. JAMA Netw Open. 2022. Source.

14. Andriole GL, et al. Effect of dutasteride on the risk of prostate cancer: REDUCE. N Engl J Med. 2010. Source.

15. Goodman PJ, et al. Long-term effects of finasteride on prostate cancer mortality. N Engl J Med. 2019;380:393–394. Source.

16. Baboudjian M, et al. Association between 5-ARIs and prostate cancer mortality: systematic review and meta-analysis. JAMA Oncol. 2023. Source.

17. Lai HH, Pontari MA, Argoff CE, et al. Male Chronic Pelvic Pain: AUA Guideline: Part II Treatment of Chronic Prostatitis/Chronic Pelvic Pain Syndrome. J Urol. 2025;214(2):127-137. doi:10.1097/JU.0000000000004565 Full 2025 guideline

18. UKCPA. Handbook of Perioperative Medicines: finasteride. Source.

19. UKCPA. Handbook of Perioperative Medicines: dutasteride. Source.

20. McConnell JD, et al. The effect of finasteride on the risk of acute urinary retention and the need for surgical treatment among men with benign prostatic hyperplasia (PLESS). N Engl J Med. 1998;338(9):557–563. Source.

21. Thompson IM, Goodman PJ, Tangen CM, et al. The influence of finasteride on the development of prostate cancer. N Engl J Med. 2003;349(3):215-24. Source.

22. Kaplan SA, McConnell JD, Roehrborn CG, Meehan AG, Lee MW, Noble WR, et al. "Combination Therapy With Doxazosin and Finasteride for Benign Prostatic Hyperplasia in Patients With Lower Urinary Tract Symptoms and a Baseline Total Prostate Volume of 25 ml or Greater." J Urol. 2006;175(1):217-220. doi:10.1016/S0022-5347(05)00041-8

23. Goueli R, Badlani GH, Welliver C, et al. "Management of Lower Urinary Tract Symptoms Attributed to Benign Prostatic Hyperplasia: AUA Guideline (2026) Part I: Presentation and Evaluation." J Urol. 2026;216(2):143-151. doi:10.1097/JU.0000000000005097

24. European Association of Urology. "EAU Guidelines on the Management of Non-neurogenic Male LUTS: Follow Up." EAU Guidelines Office; 2026. Source

25. CIALIS (tadalafil) tablets. US prescribing information. Revised March 2026. Source

26. D’Amico AV, Roehrborn CG. "Effect of 1 mg/day finasteride on concentrations of serum prostate-specific antigen in men with androgenic alopecia: a randomised controlled trial." Lancet Oncol. 2007;8(1):21–25. doi:10.1016/S1470-2045(06)70981-0

27. Lee KS, Yoo JW, Kim DH, et al. "A prospective, randomized, open-label, parallel trial comparing the efficacy of α-blocker or 5α-reductase inhibitor withdrawal to continued combination therapy on the maintenance of lower urinary tract symptoms in men with benign prostatic hyperplasia." Prostate. 2024;84(4):403–413. doi:10.1002/pros.24663

28. Lin VCH, Liao CH, Kuo HC. "Progression of lower urinary tract symptoms after discontinuation of 1 medication from 2-year combined alpha-blocker and 5-alpha-reductase inhibitor therapy for benign prostatic hyperplasia in men--a randomized multicenter study." Urology. 2014;83(2):416–421. doi:10.1016/j.urology.2013.09.036

29. GEMTESA (vibegron) tablets: US prescribing information. DailyMed. Official label.

30. Hehir CM, Calpin GG, Cullivan O, Daly GR, Dowling GP, Davis NF. "The role of 5-alpha reductase inhibitors in transurethral resection of the prostate: a meta-analysis of randomised controlled trials." BJU Int. 2026;137(3):430-443. doi:10.1111/bju.70117