Can finasteride (a 5-alpha-reductase inhibitor) cause erectile dysfunction?

Finasteride and Erectile Dysfunction

Finasteride is associated with a 2-4% increased risk of erectile dysfunction compared to placebo, with the highest risk occurring in the first year of treatment and potentially persisting after discontinuation in some patients. 1, 2

Evidence of Sexual Side Effects

Finasteride, a 5-alpha-reductase inhibitor (5-ARI), inhibits the conversion of testosterone to dihydrotestosterone (DHT) and is used primarily for:

• Benign prostatic hyperplasia (BPH)
• Male androgenetic alopecia (hair loss)

Incidence and Timing of Erectile Dysfunction

The clinical evidence consistently shows:

• First year of treatment: Highest incidence of erectile dysfunction

  • 8.1% with finasteride vs 3.7% with placebo 2
  • Relative risk of 1.71 (95% CI: 1.11-2.65) across multiple studies 1

• Years 2-4: Incidence decreases but remains statistically significant

  • 5.1% with finasteride vs 5.1% with placebo 2
  • Sexual dysfunction decreases over time but remains higher than placebo 1

Dose-Related Effects

• The 5mg dose (used for BPH) and 1mg dose (used for hair loss) both showed approximately a 50% decrease in PSA at 1 year 1
• Sexual side effects appear to be more common with the higher dose used for BPH than the lower dose used for hair loss 3

Mechanism and Risk Factors

The sexual side effects are likely related to:

• Reduction in DHT levels
• Altered androgen signaling in genital tissues
• Possible neurosteroid effects

Risk factors that may increase likelihood of experiencing erectile dysfunction:

• Advanced age
• Pre-existing sexual dysfunction
• Psychological factors (nocebo effect)

Nocebo Effect Consideration

One important study demonstrated that when patients were specifically informed about potential sexual side effects:

• 43.6% reported sexual side effects when informed beforehand
• Only 15.3% reported sexual side effects when not specifically informed 4

This suggests a significant nocebo component to the reported sexual dysfunction.

Persistence of Sexual Side Effects

While most guidelines indicate that sexual side effects are reversible, some research suggests:

• In a small percentage of patients, sexual dysfunction may persist despite discontinuation of the medication 5, 6
• In one study, 96% of subjects with persistent sexual side effects continued to experience them at reassessment 14 months later 5

However, these studies have significant limitations including selection bias and lack of hormone level measurements.

Clinical Recommendations

1. Patient counseling: Discuss the potential risk of erectile dysfunction (2-4% higher than baseline) before initiating therapy

2. Monitoring: Assess sexual function at baseline and follow-up visits

3. Risk-benefit assessment:

   • For BPH: Consider the benefit of reduced risk of acute urinary retention (2.3% absolute risk reduction) and surgical intervention (1.6% absolute risk reduction) 1
   • For hair loss: Weigh cosmetic benefits against potential sexual side effects

4. Alternative options:

   • For BPH: Alpha blockers may be preferred in patients concerned about sexual side effects
   • For hair loss: Topical treatments may be considered as first-line options

5. Management of erectile dysfunction:

   • If erectile dysfunction develops, consider discontinuing finasteride
   • Standard ED treatments may be effective while continuing finasteride if benefits outweigh risks

Important Caveats

• The magnitude of sexual dysfunction effect is relatively small compared to natural sources of variability (3.21 points on a 0-100 scale for finasteride vs. 1.26 points for each year of aging) 1
• Discontinuation rates due to sexual adverse events are approximately 3.7% for finasteride vs 2.1% for placebo 2
• Other sexual side effects include decreased libido, decreased ejaculate volume, and ejaculation disorders 1, 2

In conclusion, while finasteride does increase the risk of erectile dysfunction, the absolute risk increase is modest, tends to decrease over time, and should be weighed against the clinical benefits in each individual case.