One hundred and seventy four (81% of all) pathologically confirmed new incident cases of female breast cancer identified from a medical center in Taipei from February, 1993 to June, 1994 were selected as the case group. Four hundred and fifty three inpatient controls who were without obstetric-gynecological, breast, or malignant diseases were individually matched for each case by age and date of admission. Information was obtained through direct interview and review of medical records. Conditional logistic regression was used to estimate the effects of each risk factor. After adjusting for education level, body mass index, age at menarche and first full-term pregnancy, parity, menopausal status and age at menopause, lifetime lactation, use of lactation inhibition hormones, and family history of breast cancer, breast cancer risk significantly elevated in use of OC before 25 years old and before 1971. In stratified analysis, significantly higher risk were found in OC use before 25 years old and in duration of use less than one year among post-menopausal subjects. Our results support the notion that OC use in early life for younger women and in early calendar years increase breast cancer risk.
PIP: The effects of oral contraceptive (OC) use on the risk of breast cancer were investigated in a hospital-based case-control study conducted in Taiwan--a country with low use of OCs and a low incidence of breast cancer. 174 consecutive new incident cases of breast cancer diagnosed at National Taiwan University Hospital in Taipei in a 16-month period in 1993-94 and 435 age-matched controls admitted to the same hospital during the study period with nongynecologic, nonmalignant conditions were enrolled. 25 cases (14.4%) and 47 controls (10.4%) had ever used OCs; 9 cases (5.2%) and 15 controls (3.3%) had used the pill for 5 years or more. The risk of breast cancer was moderately elevated in OC users who started pill use before 25 years of age or before 1971, when the hormonal content of OCs was stronger. After adjustment for educational level, body mass index, ages at menarche and first full-term pregnancy, parity, menopausal status and age at menopause, lactation, family history of breast cancer, and use of female sex hormones other than OCs, the adjusted odds ratio (OR) for OC use was 1.7 (95% confidence interval (CI), 0.9-3.2). The adjusted OR for OC use before 25 years of age vs. never use was 3.4 (95% CI, 1.2-9.7). The adjusted OR for OC use before 1971 compared with never use was 3.2 (95% CI, 1.2-8.9). In premenopausal women, the adjusted OR for age at first use under 25 years of age compared with never use was 5.8 (95% CI, 1.5-22.1) and the adjusted OR for duration of OC use of 5 or more years vs. never use was 3.5 (95% CI, 0.9-14.3). For postmenopausal women, the adjusted OR of duration of OC use less than 1 year vs. never use was 7.5 (95% CI, 1.1-50.1).