The use of oral contraceptives confers long-term protection against ovarian cancer, and the longer birth control pills are used, the greater their protective effect, according to an analysis of 45 epidemiologic studies from around the world.
The findings suggest that oral contraceptives (OCs) have prevented about 200,000 ovarian cancers and 100,000 deaths from the disease worldwide during the past half century. In coming decades, the number of cancers prevented will rise to an estimated 30,000 per year, according to the Collaborative Group on Epidemiological Studies of Ovarian Cancer, led by Dr. Valerie Beral of the Cancer Research UK Epidemiology Unit at Oxford (England) University. (Lancet 2008;371:303–14).
Noting that the findings are not unexpected, the editors of the Lancet called the study “impressive and compelling.” The findings “set a new standard in primary prevention for a deadly cancer and have important public health implications,” they wrote in an editorial (Lancet 2008;371:275).
The collaborative analysis “brings unequivocal good news,” Dr. Eduardo L. Franco and Dr. Eliane Duarte-Franco wrote in a commentary (Lancet 2008;371:277–8). “Women and their health-care providers are once again at a balancing act of judging risks versus benefits.”
The analysis included 13 prospective studies, 19 case-control studies with population controls, and 13 case-control studies with hospital controls. Conducted in 21 countries, the studies included 23,000 women with malignant epithelial or nonepithelial ovarian cancer and more than 87,000 controls without ovarian cancer who had not undergone bilateral oophorectomy. On average, the cancers were diagnosed in 1993, and the mean age at diagnosis was 56 years.
Overall, 31% of the women with ovarian cancer and 37% of the controls had used OCs for an average of 4.4 and 5 years, respectively. Ovarian cancer diagnoses, on average, were made about 20 years after the women first used OCs.
Each of the three types of study design yielded a highly significant reduction in the relative risk of ovarian cancer in OC users compared with never users. “The overall relative risk decreased by 20% for each 5 years of use,” the authors wrote. Women who had used OCs for about 15 years had a 50% risk reduction.
“The proportional declines in relative risk per 5 years [of OC] use were 29% for those whose use had ceased less than 10 years previously, 19% for use ceased 10–19 years previously, and 15% for use ceased 20–29 years previously,” they reported.
In high-income countries, 10 years of OC use reduced ovarian cancer incidence before age 75 from 12 per 1,000 women to 8 per 1,000, and mortality from 7 per 1,000 women to 5 per 1,000, the researchers estimated.
There was limited evidence that OC use prevents mucinous ovarian cancer, which accounts for fewer than 15% of all cases, commented Dr. Franco of the department of oncology and epidemiology, McGill University, Montreal, and Dr. Duarte-Franco of the Institut National de Santé Publique du Québec, also in Montreal.
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