The 5-alpha reductase inhibitor (5ARI) drug class includes two FDA-approved medications: dutasteride and finasteride. These drugs inhibit the conversion of testosterone to dihydrotestosterone (DHT), which drives both prostate enlargement and androgenetic alopecia. Dutasteride is a dual inhibitor (Type 1 + Type 2), while finasteride selectively inhibits Type 2. Only finasteride carries an FDA hair loss indication; using dutasteride for hair loss is off-label.
5ARI Class Comparison
| Feature | Dutasteride | Finasteride |
|---|---|---|
| 5AR isoforms | Type 1 + Type 2 | Type 2 only |
| Serum DHT reduction | 94% at 0.5 mg (median, 1 year) | ~70% at 5 mg; ~65% at 1 mg |
| FDA: BPH | 0.5 mg (Avodart) | 5 mg (Proscar) |
| FDA: hair loss | Not approved | 1 mg (Propecia) |
| Terminal half-life | ~5 weeks at steady state | ~5-6 hours (about 8 hours over age 70) |
Class-Wide Safety Considerations
- Both drugs reduce PSA by approximately 50%, which must be accounted for in prostate cancer screening
- Both are contraindicated in women who are or may become pregnant, because of the risk of abnormal external genitalia in a male fetus. The 2011 Avodart label states this as Pregnancy Category X
- Sexual side effects (decreased libido, erectile dysfunction, ejaculation disorders) are a class effect, reported in as many as 3.4% to 15.8% of men across the reviewed literature. The current dutasteride label states these reactions may persist after treatment is stopped, and that the drug’s role in that persistence is unknown
- Blood donation deferrals apply: the American Red Cross requires a 6-month wait after the last dose of dutasteride and a 1-month wait after finasteride (eligibility criteria)
Dutasteride vs finasteride · How dutasteride works · All studies
Sources
- Hirshburg JM, Kelsey PA, Therrien CA, Gavino AC, Reichenberg JS. Adverse effects and safety of 5-alpha reductase inhibitors (finasteride, dutasteride): a systematic review. J Clin Aesthet Dermatol. 2016;9(7):56-62. PubMed 27672412
- FDA. Avodart (dutasteride) Prescribing Information, Sections 4, 12.2 and 12.3. NDA 021319, and the current revision for the persistence footnote.
- FDA. Propecia (finasteride 1 mg) Prescribing Information, Sections 12.2 and 12.3, and Proscar (finasteride 5 mg), Section 12.2.