PROGESTOGEN-ONLY ORAL-CONTRACEPTIVES AND RISK OF BREAST-CANCER IN NEW-ZEALAND

Little is known about the influence of progestogen-only oral contracep tives on a woman's risk of breast cancer. This issue was examined in a national population-based case-control study in New Zealand. A total of 891 women aged 25 to 54 years with a first diagnosis of breast canc er and 1,864 control subjects, randomly selected from the electoral ro lls, were interviewed. Use of progestogen-only pills was reported by 8 .7 percent of all control subjects (and by 17.3 percent of those aged 25 to 34 years). The relative risk (RR) of breast cancer in women who had ever used progestogen-only pills was estimated to be 1.1 (95 perce nt confidence interval [CI] = 0.73-1.5). In women aged 25 to 34 years, the RR was 2.3 (CI = 1.2-4.3). Women who had started using progestoge n-only pills within the last 10 years were at increased risk of breast cancer (RR = 1.6, CI = 1.0-2.4), whereas those who had first used the m earlier were at significantly reduced risk (RR = 0.44, CI = 0.22-0.9 0). These findings are similar to results for depot medroxyprogesteron e acetate, and a possible analogy with the influence of pregnancy is a lso suggested.