Cancer Research Lab

Oral contraceptives with low estrogen and progestin are more effective in lowering the risk of ovarian cancer than older formulations, according to investigators at the University of Hawaii in Honolulu.

The protective benefits of oral contraceptive pills have long been recognized, Dr. Galina Lurie and her associates note in their report, published in the March issue of Obstetrics & Gynecology. However, over the last 30 years the doses of hormone have decreased to reduce the frequency of side effects. Research conducted to determine how the potency changes have affected ovarian cancer risk has yielded inconsistent results.

To clarify these issues, Dr. Lurie’s group conducted a population-based case-control study in Hawaii and Los Angeles. Included were 745 women diagnosed with epithelial ovarian carcinoma between 1993 and 2005. The 943 randomly selected control subjects were matched to cases by age and ethnicity.

Data were collected by standard questionnaires, and interviewers used photo albums to help subjects identify the specific oral contraceptive pills they had used.

The researchers defined the estrogen potency by the dose of ethinyl estradiol equivalents. Progestational activity was expressed in milligrams of norgestrel equivalent. The cutoff point between high and low estrogen potency was 0.035 mg of ethinyl E2 equivalents; for progestin, it was 0.3 mg of norgestrel equivalents.

Overall, use of oral contraceptive pills was associated with a 50% reduction in risk of epithelial ovarian cancer, the investigators report.

However, compared with never-users, the odds ratio for ovarian carcinoma was 0.62 for subjects using high estrogen and high progestin formulations compared with 0.19 for those taking pills with low potency for both hormones.

“Up to 42% of ovarian cancers might have been avoided if all women used some form of combined oral contraceptive pills,” Dr. Lurie and her associates indicate. “An estimated 73% of ovarian cancers might have been avoided if all women used oral contraceptive pill formulation of low estrogen and low progestin.”

The researchers also evaluated the relationship between use of a single progestin, norethindrone, and ovarian cancer risk among 76 cases and 129 controls. The odds ratios ranged from 0.13 for women who used 0.4 or 0.5 mg norethindrone, to 0.89 for those who used 2 mg daily. However, among women who used 10 mg daily, the risk of ovarian cancer was actually increased (odds ratio, 1.23).