5-Alpha-Reductase Inhibitors Reduce DHT in the Prostate
The medications that reduce dihydrotestosterone (DHT) in the prostate are finasteride and dutasteride, both 5-alpha-reductase inhibitors (5-ARIs). 1
Mechanism and Comparative Efficacy
Finasteride
- Selectively inhibits 5-alpha-reductase type 2, the predominant isoenzyme in prostate tissue 2
- Reduces serum DHT by approximately 70% and intraprostatic DHT by 85-90% 2
- Lowers PSA by 50% after 12 months of therapy 1
- Approved dose: 5 mg daily for BPH (1 mg for male pattern baldness) 1
Dutasteride
- Inhibits both type 1 and type 2 isoforms of 5-alpha-reductase 1
- Provides superior DHT suppression compared to finasteride 3
- Reduces intraprostatic DHT by 83% at 2 weeks, 90% at 1 month, and 92-93% at 3-4 months 3
- Reduces serum DHT by >90% following 1 year of treatment 4
- Approved dose: 0.5 mg daily 3
Clinical Evidence
Dutasteride demonstrates more complete and consistent DHT suppression than finasteride due to its dual inhibition mechanism 1, 3. In men with BPH or prostate cancer, dutasteride achieved near-maximal suppression of both serum and intraprostatic DHT levels at all assessed timepoints 3.
Both medications reduce prostate volume and improve BPH symptoms, though the DHT decrease is accompanied by a reciprocal increase in testosterone levels. Importantly, even with this testosterone increase, intraprostatic testosterone levels in dutasteride-treated patients remained lower than intraprostatic DHT levels in untreated controls 3.
Important Clinical Considerations
PSA Monitoring
- Finasteride requires a multiplier of 2 for PSA interpretation after 12 months 1
- The PSA suppression effect is variable before 12 months and may exceed 50% at 3 years 1
- No consistently uniform PSA multiplier exists for dutasteride due to inter-individual variability 1
Adverse Effects
Both medications cause similar side effects 1:
- Decreased libido (statistically significant but modest magnitude)
- Decreased ejaculate volume
- Erectile dysfunction (improves over time but remains statistically significant)
- Gynecomastia
The sexual dysfunction associated with finasteride showed a mean difference of only 3.21 points on a 0-100 scale compared to placebo—smaller than the natural variability from aging (1.26 points per year) 1.
Treatment Duration
Based on the Prostate Cancer Prevention Trial (PCPT), finasteride was administered for 7 years to demonstrate efficacy in prostate cancer prevention, suggesting long-term treatment is necessary for sustained benefit 1.