Adverse Effects of Oral Finasteride
Approximately 6-7% of finasteride users discontinue treatment due to adverse effects, with sexual side effects occurring in 2-4% more patients compared to placebo, though overall discontinuation rates (including all causes) are similar between finasteride and placebo at approximately 15%. 1
Sexual Adverse Effects
The most commonly reported adverse effects are sexual in nature, with small but statistically significant increases over placebo:
First Year of Treatment
- Erectile dysfunction/impotence: 8.1% with finasteride vs 3.7% with placebo (4.4% absolute increase) 2
- Decreased libido: 6.4% with finasteride vs 3.4% with placebo (3.0% absolute increase) 2
- Decreased ejaculate volume: 3.7% with finasteride vs 0.8% with placebo (2.9% absolute increase) 2
- Ejaculation disorder: 0.8% with finasteride vs 0.1% with placebo 2
Years 2-4 of Treatment
Sexual side effects decrease substantially over time, with no significant difference between finasteride and placebo groups for impotence (5.1% vs 5.1%) and decreased libido (2.6% vs 2.6%) after the first year 2. Decreased ejaculate volume persists at 1.5% vs 0.5% in placebo 2.
Meta-Analysis Data
A 2019 systematic review of 15 randomized controlled trials (4,495 subjects) found finasteride carried a 1.66-fold relative risk of sexual dysfunction (95% CI 1.20-2.30) compared to placebo 3. A 2010 systematic review showed a 2.22-fold increased risk of erectile dysfunction (95% CI 1.03-4.78), with a number needed to harm of 82.1 patients 4.
Endocrine Effects
- Gynecomastia: 0.5% in year 1, increasing to 1.8% in years 2-4 with finasteride vs 0.1% and 1.1% with placebo, respectively 2
- Breast tenderness: 0.4% in year 1,0.7% in years 2-4 with finasteride 2
Across all RCTs, finasteride shows a 2-4% absolute increase in gynecomastia compared to placebo 1.
Overall Discontinuation Rates
The combined discontinuation rate specifically due to adverse events is approximately 6-7% for both finasteride and placebo groups 1. Sexual adverse effects account for 3.7% of discontinuations in the finasteride group vs 2.1% in placebo 2.
The overall discontinuation rate for any reason (including loss to follow-up) is approximately 15% in both finasteride and placebo arms 1.
Other Adverse Effects
- Rash: 0.5% with finasteride vs 0.2% with placebo 2
- No significant differences in other systemic adverse effects 2
Important Clinical Context
Magnitude of Sexual Dysfunction
The clinical significance of sexual side effects must be contextualized: on a 0-100 scale measuring sexual dysfunction, finasteride caused a mean difference of only 3.21 points, compared to 1.26 points for each additional year of age 1. This suggests the effect is smaller than natural age-related variability 1.
Reversibility
All sexual adverse events were reversed upon discontinuation of therapy, and many resolved in patients who continued treatment 5.
Dose Considerations
For the 1 mg dose used in androgenetic alopecia (versus 5 mg for BPH), sexual dysfunction rates in clinical studies ranged from 3.8% vs 2.1% in placebo 5, with one study reporting sexual side effects in 17.1% of patients taking dutasteride 0.5 mg after failing finasteride 6.
Key Clinical Pitfall
Do not dismiss patient concerns about sexual side effects, even though the absolute magnitude is small and often resolves with continued treatment or upon discontinuation. The relative risk increase is real and statistically significant, affecting approximately 1 in 25-50 patients beyond baseline rates 3, 4.