Oral contraceptive (OC) use is a risk factor for premenopausal breast cancer, according to pooled data published in the journal Mayo Clinic Proceedings.

However, even for women most at risk - parous women (i.e., those who have given birth) and who used OCs for at least 4 years before their first full-term pregnancy - “the absolute risk ...is very small,” Dr. James R. Cerhan, from Mayo Clinic College of Medicine in Rochester, Minnesota, emphasizes in an accompanying editorial.

Lead author Dr. Chris Kahlenborn, from Altoona Hospital in Pennsylvania, and associates note in their paper that the rate of breast cancer among premenopausal women in the US has increased considerably over the last few decades.

Kahlenborn’s group pooled data from 34 case-control studies of breast cancer and OC use, published since 1980, in which subjects were younger than 50 years old or premenopausal.

Overall, OC use was associated with a slight increase in breast cancer risk (odds ratio, 1.19). For nulliparous women (i.e., those who have never given birth) and who ever used OCs and nulliparous women who used OCs for at least 4 years, corresponding ORs were 1.24 and 1.29.

Among parous women, the OR for ever use was 1.29, while the OR for OC use before a first full-term pregnancy was 1.44, higher than if OCs were used only after the first pregnancy (1.15).

While more studies are needed, the team concludes that OCs are carcinogenic. Their conclusion is bolstered by the recent classification of OCs by the World Health Organization as a group 1 carcinogen.

In a Mayo Clinic press release, Kahlenborn advises doctors to provide more detailed information to women about OCs, calling it a “clear-cut informed-consent issue.”

In his editorial, Cerhan remarks that the increased risk associated with OC use decreases over time, so that by 10 years after last use, there is not significant association with breast cancer.

He advises that consideration of the risks of OC use should also take into account the benefits. For example, OCs decrease the risk of ovarian and endometrial cancer, he writes, and they have other benefits as well including relief of menstrual disorders, pelvic inflammatory disease, ovarian cysts, and improved bone mineral density.

Cerhan concludes that the study by Kahlenborn’s group highlights the need for more research about recent OC use and for other modifiable factors.

SOURCE: Mayo Clinic Proceedings, October 2006.