Abstract
A potential link has been suggested between dispensed finasteride and increased risk of male breast cancer (MBC). Due to the rare occurrence of MBC, it remains to be established if such a relationship exists. The purpose of this study was to combine nationwide registers in four countries to assess the potential association between dispensed finasteride and MBC. A cohort of all males with dispensed finasteride in Denmark, Finland, Norway, and Sweden (1,365,088 person years) was followed up for up to 15 years for breast cancer, and compared to a cohort of males unexposed to finasteride. Individual‐level register data included country, dates of dispensed finasteride, MBC diagnosis, and death. Incidence rate ratios (IRRs) were estimated using a generalized linear model with a Poisson distribution. An increased risk of MBC was found among finasteride users (IRR = 1.44, 95% confidence interval [95% CI] = 1.11–1.88) compared to nonusers. The IRR increased to 1.60 (95% CI = 1.20–2.13) when users in Norway and Sweden with short follow‐up time were excluded. The highest IRR was seen among men with medium duration of dispensed finasteride, medium accumulated consumption of finasteride, and among men with first dispensed finasteride prescription 1–3 years prior to diagnosis. The analyses suggested possible ascertainment bias and did not support a clear relationship between dispensed finasteride and MBC. In conclusion, a significant association between dispensed finasteride and MBC was identified. However, due to limited data for adjustment of potential confounding and surveillance bias in the present study, further research is needed to confirm these results.
Highlights
Finasteride is used primarily to relief symptoms from benign prostatic hyperplasia (5 mg pills) and to treat androgenetic alopecia (1 mg pills)
This corresponded to a crude incidence rate (IR) of 0.80 male breast cancer (MBC) cases per 100,000 person years (PY) (Table 1)
(ref) 1.08–2.37 0.46–3.35 0.12–1.95 1.13–2.74 0.26–2.56 0.53–2.02 0.58–2.42 1.20–3.65 0.65–2.90 0.42–1.73 dispensed finasteride prescriptions, and years after first dispensed finasteride prescription did not change when individuals with their first dispensed finasteride prescription in the first year of the prescription registers were excluded (Table 3). In this population-based register study, we found a statistically significant 44% increased risk of MBC among men having finasteride prescribed compared to nonusers
Summary
Finasteride is used primarily to relief symptoms from benign prostatic hyperplasia (5 mg pills) and to treat androgenetic alopecia (1 mg pills). Use of finasteride increases estradiol levels, which may cause gynecomastia [2]. The crude incidence rate of MBC in persons treated with finasteride was 7.8 per 100,000 PY (95% confidence interval [95% CI] = 3.7–16.4), which was higher not statistically different from the MBC rate 3.8 per 100,000 PY (95% CI = 1.2–11.9) in the unexposed group (P = 0.33). On this basis, the review concluded that it could not be excluded that finasteride could be associated with an increased risk of MBC [6]. Two recent studies found no significant association between finasteride use and MBC [10, 11]
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