Women who have used injectable or oral birth control in the past are at a significantly higher relative risk of invasive breast cancer, but they are at significantly lower risk of ovarian cancer, according to a new study based on black women in South Africa. As more time passed after a woman stopped using the contraceptives, her increased risk diminished.
The study, published in PLoS Medicine, pulled self-reported data from 5,702 participants with newly diagnosed invasive breast, cervical, ovarian or endometrial cancers. There were 1,492 women in the study who served as controls. They had other types of cancers, including colon, rectal and non-Hodgkin lymphoma, which are not influenced by contraceptive use.
Among the participants, 26 percent of women had used injectable hormones and 20 had used pills. After adjusting for confounding factors, including age, education, smoking and number of sexual partners, researchers found women were 1.7 times more likely to develop breast cancer and 1.4 times more likely to get cervical cancer than women who had never taken the contraceptives.
About 50 percent of women with breast cancer had used oral or injectable contraceptives whereas 26 percent of women with ovarian cancer had used the contraceptives and 17 percent with endometrial cancer had used them.
In women who had used birth control pills or injectable contraceptives, the cancer risk diminished with time after a woman's last use of the birth control, the authors wrote.
Injectable contraceptives are very common among black women in South Africa, the authors noted. In the U.S., birth control pills are a more commonly used form of female contraceptive.
Hormone medications are among the most commonly prescribed and taken medications in the world. About 9 percent of women ages 15 to 49 took oral contraceptive pills and 4 percent used injectable contraceptives or implants in 2007, according to a 2009 United Nations report. Combined injectable contraceptives provide a monthly dose of hormones to prevent pregnancy in the same way that oral contraceptives do. Brand names include Cyclofem and Novafem.
But despite the numbers, Dr. Diane Harper, director of the Gynecologic Cancer Prevention Research Group at University of Missouri-Kansas City, said women should not necessarily be deterred from using oral or injectable hormones, in South Africa or anywhere else.
The authors of the study in South Africa did not return ABC News' requests for comment.
"The very large benefit of contraceptives for women of reproductive age in preventing maternal deaths due to childbearing are largely overlooked by this study," said Harper. "Any increased risk of breast or cervical cancer due to short-term use of contraceptives must be weighed by the quality of the data coming from the self-reports, by the large number of deaths prevented during childbearing, and by the multiple factors in addition to hormone exposure that play into pre-menopausal versus post-menopausal breast cancer and cervical [cancer]."