• Isla S Mackenzie, Thomas M Macdonald, Alastair Thompson, Steve Morant, and Li Wei.
• Medicines Monitoring Unit, University of Dundee, Dundee DD1 9SY, UK. i.s.mackenzie@dundee.ac.uk
• BMJ. 2012 Jan 1;345:e4447.

ObjectiveTo investigate whether exposure to spironolactone treatment affects the risk of incident breast cancer in women over 55 years of age.DesignRetrospective, matched cohort study.SettingGeneral Practice Research Database, a primary care anonymised database representative of the general population in the United Kingdom.Participants1,290,625 female patients, older than 55 years and with no history of breast cancer, from 557 general practices with a total follow-up time of 8.4 million patient years. We excluded patients with poor quality data and those with no contacts with their general practitioner after their current registration date.InterventionExposed cohort included women who received at least two prescriptions of spironolactone after age 55 years, who were followed up from the first prescription (index date). We randomly selected two unexposed female controls for every exposed patient, matched by practice, year of birth, and socioeconomic scores (if information was available), and followed up from the same date.Main Outcome MeasureNew cases of breast cancer, using Read codes to confirm diagnoses.ResultsIndex dates for study patients ranged from 1987 to 2010, and 29,491 new cases of breast cancer were recorded in the study population (incidence rate 0.35% per year). The exposed cohort of 28,032 patients and control cohort of 55,961 patients had unadjusted incidence rates of 0.39% and 0.38% per year, respectively, over a mean follow-up time of 4.1 years. Time-to-event analysis, adjusting for potential risk factors, provided no evidence of an increased incidence of breast cancer in patients exposed to spironolactone (hazard ratio 0.99, 95% confidence interval 0.87 to 1.12).ConclusionsThese data suggest that the long term management of cardiovascular conditions with spironolactone does not increase the risk of breast cancer in women older than 55 years with no history of the disease.

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