By Kathleen Doheny
THURSDAY, July 31, 2014 (HealthDay News) -- Birth control pills containing high doses of estrogen, along with some other formulations, may increase the risk of breast cancer in women under 50, new preliminary research suggests.
"There are numerous oral contraceptive formulations," explained lead researcher Elisabeth Beaber, a staff scientist at the Fred Hutchinson Cancer Research Center in Seattle. "Some of these formulations increase breast cancer risk while other formulations do not raise risk."
This study was designed to find a possible link between oral contraceptive use and breast cancer risk in younger women. But, it wasn't designed to prove that birth control pills definitively cause the increased risk. However, the researchers did take into account other factors that increase breast cancer risk, such as family history. In addition, they found that the link was slightly stronger -- though not statistically significant -- for breast cancers termed estrogen-receptor positive. This type of cancer needs estrogen to grow, which might help explain why high-dose estrogen pills elevated risk.
The researchers also found variations in risk among different formulas, with low-dose estrogen pills appearing safest. "Recent use of oral contraceptives containing low-dose estrogen [20 micrograms ethinyl estradiol] did not appear to increase breast cancer risk," Beaber said.
These lower-dose pills account for an increasing number of prescriptions written today, Beaber said.
Which formulations seemed to raise the risk of breast cancer? High-dose estrogen pills -- those containing 50 micrograms ethinyl estradiol or 80 micrograms mestranol -- were associated with nearly a three-fold higher risk of breast cancer, she said. Triphasic combination pills with 0.75 milligrams of norethindrone were linked to more than a three-fold higher risk of breast cancer, according to Beaber.
Pills with ethynodiol diacetate -- a progestin -- appeared to increase the risk of breast 2.6 fold, Beaber said.
Risks seemed lower with moderate-dose estrogen pills -- those with 30 to 35 micrograms of ethinyl estradiol or 50 micrograms mestranol were linked to a 1.6 times higher risk of breast cancer.
How can a woman tell if she's taking a formulation linked to a higher risk? "The specific doses and types of hormones used in oral contraceptives are included in packaging information," Beaber said.
The study, funded by the National Cancer Institute, was published Aug. 1 in the journal Cancer Research.
Beaber stressed that the study results need to be confirmed before any recommendations can be made to women. The results are based on data about recent oral contraceptive use who were diagnosed with breast cancer and nearly 22,000 healthy women who served as the comparison group. The women were all between the ages of 20 and 49.
The researchers used electronic pharmacy records to gather information on prescriptions filled and information on formulas. The study looked at the years 1990 through 2009.
The researchers evaluated the risks of breast cancer in women who had taken birth control pills in the past year compared to former or never users. They then looked at risk with the specific formulas of birth control pills.
The study results suggest that the lower-dose estrogen pills, which became popular in the 1990s, are not a problem, said Dr. Courtney Vito, a breast surgeon and assistant clinical professor of surgical oncology at the City of Hope Comprehensive Cancer Center in Duarte, Ca.
While the researchers made a good attempt to answer the question about risks associated with different birth control formulas, ''the study has some flaws that are inherent in this type of study design," Vito said. For instance, they could not control for all factors that could increase breast cancer risk.
And, as the researchers also noted, the duration of time they evaluated was relatively brief.
The best advice for women taking birth control pills? " Talk to your doctor about considering a lower dose estrogen birth control pill that does not have the higher-risk progesterone in it," Vito said.
"Although these results suggest an increased risk of breast cancer, the many established health benefits associated with oral contraceptive use... must also be considered when making individual choices," wrote the study's authors.
The study's authors also pointed out that any potential increased risk likely goes down when a woman stops using birth control pills.