Oral contraceptives and risk of breast cancer. A historical prospective case-control study.

ABSTRACT

BACKGROUND: The goal of this study is to quantify the risk of breast cancer related to use of oral contraceptives while adjusting for known risk factors, e.g. age at first delivery, number of children, and family history, as well as minimising memory bias. METHODS: A historical prospective study design was used. Data collected over a period of 25 years in the screening program of a cancer detection clinic for women aged 25-69 years were utilised. Information on breast cancer among those attending the screening program was searched for in the national cancer registry. Women with breast cancer were matched on date of birth with on average 5.3 control women who were still alive when the diagnosis was made. Mothers and sisters of cases and controls were identified through a national genealogy registry. RESULTS: The odds ratio of developing breast cancer among women with a first degree relative with breast cancer was about 2, but for those ever using oral contraceptives it was 0.92 and even lower (0.50-0.75) if the cancer was diagnosed before the age of 45 years. CONCLUSIONS: Use of oral contraceptives (OC) does not seem to increase the risk of developing breast cancer among women in Iceland.