The objective of this study is to investigate the potential association between finasteride (MK-0906) exposure and the development of breast cancer in men residing in Denmark, Sweden, Finland, and Norway from data in national registries. The primary hypothesis of this study is that the previously reported increased incidence of male breast cancer among finasteride users is explained by confounding factors.

Inclusion Criteria:

• Participant's study medical information available in population-based health and medical registries, prescription registries, and mortality registries in Denmark, Finland, Norway, or Sweden within the observation period of 1995 to 2013
• Male with breast cancer with medical information in one of the 4 country-specific registries in this study OR country- and age-matched control men without breast cancer and with medical information in one of the 4 country-specific registries
• Study participant's exposure to finasteride is available

Exclusion Criteria:

For country- and age-matched control men without breast cancer

• Previous cancer diagnosis or treatment for cancer except non-melanoma skin cancer
• Previous prostatectomy
• Finasteride or dutasteride use (dutasteride is a drug in the same class as finasteride) within first 6 months of registration in the prescription registers (new user design).

Population-based health and medical registries, prescription registries, and mortality registries in Denmark, Finland, Norway, and Sweden