Use of oral contraceptives during a woman's life-time gives substantial long-term protection against ovarian cancer, and the longer they are used, the greater the reduction in risk, according to an article in The Lancet. Use of oral contraceptives has long been known to reduce the incidence of ovarian cancer. Because ovarian cancer is not common in young women and the incidence increases with age, the public-health effect of this reduction depends mainly on how the reduced risk persists decades after oral contraceptive use ceases.

Professor Valerie Beral and colleagues from the Collaborative Group on Epidemiological Studies of Ovarian Cancer have brought together 45 epidemiological studies of ovarian cancer to prepare the Article. They studied 23,257 women with ovarian cancer, 7308 (31%) of whom had ever used oral contraceptives; and 87,303 women without ovarian cancer, of which 32 717 (37%) had ever used oral contraceptives. In women with ovarian cancer, the mean age of diagnosis was 56 years and the median year of diagnosis was 1993. Those that had used contraceptives had done so for an average of 4.4 years in the ovarian cancer group and 5.0 years in the control group.

The researchers found that in high income countries, 10 years use of oral contraceptives was estimated to reduce ovarian cancer incidence before age 75 from 12 per 1000 women to 8 per 1000, and mortality from 7 per 1000 women to 5 per 1000. For every 5000 woman-years of oral contraceptive use (e.g. if 5000 women used oral contraceptives for one year), about two ovarian cancers and one death from the disease before age 75 are prevented. Further, they found that although oestrogen doses in oral contraceptives have decreased significantly over the years (preparations in the 1960s typically contained more than double the oestrogen dose of preparations in the 1980s), there was no apparent variation in the relative risk of ovarian cancer between women whose oral contraceptive use was during the 1960s, 1970s or 1980s. The risk reduction did not vary substantially by women's ethnicity, education, age her periods began, family history of breast cancer, use of hormone replacement therapy, body-mass index, height, or consumption of alcohol or tobacco.

The authors conclude: "Use of oral contraceptives confers long-term protection against ovarian cancer. These findings suggest that oral contraceptives have already prevented some 200,000 ovarian cancers and 100,000 deaths from the disease, and that over the next few decades the numbers of cancers prevented will rise to at least 30,000 per year." In an accompanying Comment, Dr Eduardo Franco, McGill University, Montreal, Quebec, Canada, and Dr Eliane Duarte-Franco, Institut National de Santé Publique de Québec, Montreal, Quebec, Canada, say: "As for the link between oral contraceptives and ovarian cancer, today's collaborative analysis brings unequivocal good news. Women and their health-care providers are once again at a balancing act of judging risks versus benefits."