Mechanism of Action

This page provides an overview of the biological role of 5-alpha reductase and describes how 5-ARIs (dutasteride and finasteride) affect the conversion of testosterone to dihydrotestosterone (DHT). The goal is to summarize the available evidence on the underlying biochemical pathway and support clinicians in understanding where 5-ARIs act within androgen signaling.

5-ARIs are not FDA-approved for patients with prostate cancer. This page is intended for informational purposes only and is not intended to provide medical advice and should not be relied upon in that regard. Clinicians should make individual prescribing decisions for their patients based on the patient's clinical presentation and the clinician’s own medical decision-making.

5-ARIs inhibit the 5-alpha reductase enzyme, which converts testosterone to DHT.1,2 By inhibiting this enzymatic step, 5-ARIs lower DHT levels in serum and prostate tissue.1,2 Reported reductions range from 70–95%, depending on the 5-ARI, dose, and duration of use.1,2 Lower DHT levels may influence downstream androgen receptor signaling, though the nature and clinical relevance of these downstream effects vary.3 Dutasteride inhibits both type 1 and type 2 isoforms of 5-alpha reductase, while finasteride primarily inhibits the type 2 isoform.1,2

References:

1. Chislett B, et al. Transl Androl Urol. 2023;12(3):487–96. 2. Keam SJ, Scott LJ. Drugs. 2008;68(4):463–85. 3. Al-Horani RA, Patel P. Dutasteride. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; Updated March 20, 2024. Accessed January 8, 2026. https://www.ncbi.nlm.nih.gov/books/NBK603726

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