Incidence of Gynecomastia with Oral Finasteride
Gynecomastia occurs in approximately 2-4% of men taking finasteride, representing roughly a 2-fold increased risk compared to placebo. 1, 2
Specific Incidence Data by Dose and Duration
Standard 5 mg Dose (BPH Treatment)
Mid-term studies (1-2 years): Gynecomastia occurred in 2.3% of finasteride-treated patients versus 0.74% on placebo, yielding a relative risk of 3.11 1
Long-term studies (≥2 years): The PCPT trial showed gynecomastia in 4.5% of finasteride patients versus 2.8% on placebo over 7 years, with a relative risk of 1.64 1
MTOPS trial (4-6 years): Gynecomastia occurred in 2.2% of finasteride monotherapy patients versus 0.7% on placebo 3
Combined analysis: Overall incidence was 4.1% with finasteride versus 2.4% with placebo, representing a relative risk of 2.13 1
Low-Dose 1 mg (Androgenetic Alopecia Treatment)
Reported incidence: Gynecomastia ranges from 0.5-2.2% versus 0.1-1.1% with placebo 2
Breast tenderness: Affects 0.4-0.7% of patients 2
Critical Clinical Considerations
Underreporting Concerns
Only 8 cases of gynecomastia with 1 mg finasteride have been formally reported since FDA approval in 1997, suggesting significant underreporting. 4 This discrepancy between clinical trial data and post-marketing reports raises concern that patients may not be adequately informed about this risk 4
Persistence and Reversibility
Gynecomastia can persist after drug discontinuation and may become irreversible if fibrotic changes develop 4
One documented case showed persistent gynecomastia 5 months after stopping finasteride, requiring bilateral mammoplasty for resolution despite 6 months of raloxifene therapy 4
Early intervention is critical to prevent fibrosis and permanent breast tissue changes 4
Mechanism
The gynecomastia results from alterations in the estrogen/androgen ratio due to finasteride-induced decreases in circulating dihydrotestosterone levels 5 This is an expected pharmacologic consequence of 5-alpha-reductase inhibition 4
Comparison to Other Adverse Effects
For context, gynecomastia occurs less frequently than sexual side effects with finasteride:
- Erectile dysfunction: 4.2-15.8% (additional 2-4% vs placebo) 2
- Decreased libido: 3.4-10% (additional 2-4% vs placebo) 2
- Reduced ejaculate volume: 1.5-7.2% 2
Clinical Practice Implications
Physicians must counsel all patients about gynecomastia risk before prescribing finasteride, regardless of dose or indication. 4, 6 The American Society of Clinical Oncology/American Urological Association guidelines acknowledge gynecomastia as a consistent adverse effect requiring patient discussion 1
Discontinuation rates specifically due to adverse events (including gynecomastia) are approximately 6-7% for both finasteride and placebo groups, suggesting most cases are tolerable. 1, 2