Characteristics of finasteride users in comparison with nonusers: A Nordic nationwide study based on individual-level data from Denmark, Finland, and Sweden.

Purpose Published epidemiological studies on the association between finasteride use and the risk of male breast cancer have been inconclusive due to methodological limitations including a few male breast cancer cases included. Determinants of male breast cancer have been studied, but it remains unexplored whether these are also related to finasteride use and thereby constitute potential confounders. This study aimed to assess whether there are differences between finasteride users and nonusers with regard to numerous potential confounders. Methods In total, 246 508 finasteride users (>= 35 years) were identified in the prescription registries of Denmark (1995-2014), Finland (1997-2013), and Sweden (2005-2014). An equal number of nonusers were sampled. The directed acyclic graph (DAG) methodology was used to identify potential confounders for the association between finasteride and male breast cancer. A logistic regression model compared finasteride users and nonusers with regard to potential confounders that were measurable in registries and population surveys. Results Finasteride users had higher odds of testicular abnormalities (odds ratio [OR] 1.40; 95% confidence interval [CI] 1.36-1.44), obesity (1.31; 1.23-1.39), exogenous testosterone (1.61; 1.48-1.74), radiation exposure (1.22; 1.18-1.27), and diabetes (1.07; 1.04-1.10) and lower odds of occupational exposure in perfume industry or in high temperature environments (0.93; 0.87-0.99), living alone (0.89; 0.88-0.91), living in urban/suburban areas (0.97; 0.95-0.99), and physical inactivity (0.70; 0.50-0.99) compared to nonusers. Conclusions Systematic differences between finasteride users and nonusers were found emphasizing the importance of confounder adjustment of associations between finasteride and male breast cancer.