The FDA label records breast disorders, meaning breast enlargement, breast tenderness, or both, in 0.5% to 1.1% of men on dutasteride across the four 6-month intervals of the pooled BPH trials, compared with 0.1% to 0.3% on placebo. The usual explanation is the altered testosterone-to-estrogen ratio that follows DHT suppression. The label reports no evidence of increased breast disorders with increased duration of treatment. Dutasteride is FDA-approved in the United States for benign prostatic hyperplasia only; use for hair loss is off-label.
What Causes Gynecomastia on Dutasteride?
The usual explanation is that when dutasteride blocks the conversion of testosterone to DHT, more testosterone is available for conversion to estradiol (estrogen) by the aromatase enzyme, shifting the balance of estrogen to androgens in a direction that can stimulate breast tissue. That mechanism is a proposed one, not a demonstrated one: in healthy volunteers treated for 52 weeks, the FDA label reports no clinically significant change in estradiol compared with placebo, alongside a measured rise in total testosterone. The same reasoning is applied to finasteride, and no head-to-head trial establishing a difference in gynecomastia risk between the two drugs is cited in either product’s label.
Clinical Data
In the pooled BPH trials (FDA label, Table 1), breast disorders were reported by 0.5% of men on dutasteride at months 0-6, 0.8% at months 7-12, 1.1% at months 13-18 and 0.6% at months 19-24, against 0.2%, 0.3%, 0.3% and 0.1% on placebo. In the 4-year CombAT trial (Table 2), the corresponding figures for dutasteride monotherapy were 0.9%, 0.9%, 1.2%, 0.5% and 0.7% across years 1 to 4. The label records that the most common adverse reaction leading to study withdrawal was impotence, at 1%.
What to Do If You Develop Gynecomastia
- Consult your doctor: gynecomastia should be evaluated to rule out other causes
- Monitor: The label reports no evidence of increased breast disorders with increased duration of treatment, but it does not state how often breast tenderness settles on its own or how long that takes
- Consider discontinuation: The label gives no resolution timeline for breast disorders after stopping. What it does establish is that serum dutasteride remains detectable for up to 4 to 6 months after the last dose, so nothing about this drug reverses quickly
- Report new lumps: Male breast cancer appears in the label’s post-approval adverse reactions. Across the three pivotal trials there was 1 case in the dutasteride group and 1 in the placebo group, and the label states the relationship between long-term dutasteride use and male breast neoplasia is not known. Any persistent lump should be evaluated
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Sources
- FDA. Avodart (dutasteride) prescribing information, Adverse Reactions Tables 1 and 2. accessdata.fda.gov