How Finasteride Affects Benign Prostatic Hyperplasia
Finasteride effectively treats BPH by shrinking the prostate gland by 15-25% within 6 months, improving urinary symptoms by an average of 3 points on the AUA Symptom Index, and most importantly, reducing the risk of acute urinary retention by 67% and the need for BPH-related surgery by 64% compared to placebo. 1, 2
Mechanism of Action
Finasteride inhibits the type 2 isoenzyme of 5-alpha-reductase, which converts testosterone to dihydrotestosterone (DHT) in the prostate. 1 This results in:
- Approximately 70% reduction in prostatic DHT levels 3
- 15-25% reduction in prostate volume within 6 months 3, 4
- Increased peak urinary flow rate (average 1.7 mL/sec improvement) 1, 5
- 50% reduction in serum PSA levels after 1 year 3, 4
Clinical Benefits and Symptom Improvement
Symptom relief occurs gradually over 3-6 months, with maximum benefit at 6-12 months. 3 The average patient experiences:
- 3-point improvement in AUA Symptom Index, which patients perceive as clinically meaningful 1
- Long-term symptom improvements maintained for 6-10 years 3, 4
- Greater benefit in men with larger prostates (>30cc) and/or higher PSA values 1, 3, 4
Disease Modification and Prevention of Complications
The most critical benefit of finasteride is preventing disease progression and serious complications:
- 67% reduction in risk of acute urinary retention (0.8% vs 2.4% with placebo) 2
- 64% reduction in need for BPH-related surgery (2.0% vs 5.4% with placebo) 1, 2
- 67% reduction in overall clinical progression when combined with an alpha-blocker 3, 2
These benefits increase with rising prostate volume or serum PSA, as patients with larger glands face progressively higher risk of complications over time. 1
Critical Patient Selection Criteria
Finasteride is ONLY effective in patients with documented prostatic enlargement (prostate volume >30cc). 1, 3, 4
Do not prescribe finasteride for men with LUTS who do not have prostatic enlargement—it is ineffective and exposes them to unnecessary side effects. 1, 3
Combination Therapy Superiority
Combination therapy with finasteride plus an alpha-blocker (such as tamsulosin or doxazosin) is superior to either medication alone:
- 64% reduction in symptom score progression (vs 30% with finasteride alone, 46% with doxazosin alone) 2
- 79% reduction in acute urinary retention compared to alpha-blocker alone 3
- 67% reduction in need for surgery compared to alpha-blocker alone 3
The alpha-blocker provides immediate symptom relief (within 3-5 days), while finasteride provides long-term disease modification. 3
Adverse Effects
Sexual side effects are the primary concern:
- Decreased libido: 6.4% in first year (decreases to 2.6% in years 2-4) 3, 4
- Ejaculatory dysfunction: 3.7% in first year (decreases to 1.5% in years 2-4) 3, 4
- Erectile dysfunction: 4-15% 4
These effects are typically reversible and become uncommon after the first year of therapy. 1, 3 However, the FDA has acknowledged that persistent sexual dysfunction may continue after drug discontinuation in a small subset of patients. 3
Critical Monitoring Considerations
PSA levels decrease by approximately 50% after 1 year of finasteride therapy. 3, 4 When screening for prostate cancer in men on finasteride, double the measured PSA value after 1 year of therapy to accurately interpret results. 3, 4 Failure to adjust PSA interpretation can lead to delayed cancer diagnosis. 3
Common Pitfalls to Avoid
- Using finasteride in patients without prostatic enlargement (<30cc) is ineffective and exposes patients to unnecessary side effects 1, 3, 4
- Not allowing adequate time (at least 6 months) to assess effectiveness before declaring treatment failure 3, 4
- Failing to adjust PSA interpretation (doubling the value after 1 year) can lead to delayed cancer diagnosis 3, 4
- Dismissing patient concerns about persistent sexual side effects, as the FDA has acknowledged these may occur 3