Dutasteride is a dual inhibitor of both type I and type II 5-alpha reductase, resulting in more complete DHT suppression than finasteride (~95% vs ~70%). It is approved for the treatment of BPH (as monotherapy or in combination with tamsulosin as Jalyn) and is used off-label for androgenetic alopecia. Due to its very long half-life, effects persist long after discontinuation.
Reduces risk of acute urinary retention and BPH surgery2
Combination with tamsulosin (Jalyn) provides dual mechanism benefit1,2
What to watch for
Erectile dysfunction
Decreased libido
Decreased ejaculate volume
Women who are or may become pregnant (teratogenic; absorbed through skin)
Known hypersensitivity to dutasteride, other 5-alpha reductase inhibitors, or capsule components1,2
The bottom line
Evidence rating strong. Most-documented uses: greater dht suppression than finasteride, significant prostate volume reduction (25-30%), reduces risk of acute urinary retention and bph surgery. 10 sources indexed (2014–2025), with 2 interaction records on file.
The science
How it works, mechanistically.
Core mechanism
Irreversibly inhibits both type I and type II isoforms of 5-alpha reductase, the enzymes that convert testosterone to dihydrotestosterone (DHT). This dual inhibition reduces serum DHT by >90% (compared to ~70% with finasteride). The resulting profound DHT suppression leads to greater prostate volume reduction than finasteride.8
Class
5-Alpha Reductase Inhibitor / Urologic
Absorption
Fat-soluble; take with food
Dosing
Dosing & protocol.
Common range
0.5 mg once daily (as prescribed by your physician)
Recommended form
Soft gelatin capsule; swallow whole, do not open or chew (drug is irritating to oropharyngeal mucosa)
Absolute bioavailability ~60%; food does not significantly affect AUC. Capsules should be swallowed whole. Extremely long half-life (5 weeks) means drug effects persist for months after discontinuation.
Safety
Full safety detail.
Side effects
Erectile dysfunction
Decreased libido
Decreased ejaculate volume
Gynecomastia and breast tenderness
Dizziness
Ejaculation disorders
Contraindications
Women who are or may become pregnant (teratogenic; absorbed through skin)
Known hypersensitivity to dutasteride, other 5-alpha reductase inhibitors, or capsule components1,2
Dutasteride exposure is vulnerable to CYP3A-related changes, and St. John's Wort is a clinically important inducer of CYP3A4 and P-glycoprotein. Direct St. John's Wort-dutasteride coadministration data are limited, but dutasteride has documented CYP3A-mediated interaction potential and St. John's Wort has well-established induction effects. The practical concern is reduced dutasteride exposure and less complete DHT suppression during chronic BPH or alopecia treatment.
Recommendation: Avoid St. John's Wort while taking dutasteride unless the prescriber specifically approves it. Do not adjust dutasteride dosing yourself; instead, report any worsening urinary symptoms or hair-loss control after starting or stopping St. John's Wort.
Saw palmetto is used for LUTS/BPH and has proposed antiandrogenic activity that overlaps with dutasteride, a dual 5-alpha-reductase inhibitor. PubMed-indexed BPH literature directly compares Serenoa-containing therapy with dutasteride and reviews beta-sitosterol/saw-palmetto pathways, while a recent case series raises concern for persistent sexual and psychiatric symptoms after Serenoa exposure. Co-use may complicate assessment of BPH response, PSA interpretation, sexual adverse effects, and mood symptoms.
Recommendation: Do not add saw palmetto to dutasteride without telling the prescriber. Report changes in libido, erectile function, mood, breast tenderness, urinary symptoms, or PSA monitoring context, especially after starting or stopping saw palmetto.
Yu H, Chen H, Wang X et al.. Efficacy and Safety of Combination Tamsulosin and Dutasteride versus Tamsulosin Monotherapy for Benign Prostatic Hyperplasia: A Meta-Analysis. Archivos espanoles de urologia. 2025
Yang F, Hashim R, Philippou J. Tamsulosin and Dutasteride Combination Therapy for Asian Men With Moderate-to-Severe Lower Urinary Tract Symptoms Secondary to Benign Prostatic Hyperplasia: A Systematic Review of Clinical Considerations That Influence the Prescription. Journal of clinical medicine research. 2024
Zhou Z, Cui Y, Wu J et al.. Meta-analysis of the efficacy and safety of combination of tamsulosin plus dutasteride compared with tamsulosin monotherapy in treating benign prostatic hyperplasia. BMC urology. 2019
Ren J, Lai S, Jiang Z et al.. A Systematic Review and Meta-Analysis of the Effects on Dutasteride Treatment for Reducing Surgical Blood Loss during Transurethral Resection of the Prostate. Urologia internationalis. 2017
Wu XJ, Zhi Y, Zheng J et al.. Dutasteride on benign prostatic hyperplasia: a meta-analysis on randomized clinical trials in 6460 patients. Urology. 2014
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