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August 4, 2014 — Birth control pills that contain high doses of estrogen may increase the risk of breast cancer for women under 50, according to a study published in Cancer Research.

Researchers looked at data on 1,102 women aged 20-49 years old who were diagnosed with invasive breast cancer between 1990 and 2009. Women who used birth control pills within the last year were 50% more likely to have breast cancer compared to women who did not use birth control pills or only used them at least one year ago.

The link was three times higher for women who recently used birth control pills with high doses of estrogen (50-micrograms of ethinyl estradiol or 80-micrograms of mestranol). Examples include Ovral and Ovcon. Very few women take high-dose pills, and usually only if they have a medical condition.

Two types of birth control pills were associated with the highest risk. Continuin or Femulen, which contain ethynodiol diacetate, increased the risk of breast cancer by 2.6-times. Ortho 75, which is a triphasic combination pill containing 0.75 milligrams of norethindrone, increased the risk of breast cancer by 3.1-times.

Low-dose estrogen pills (20-micrograms of ethinyl estradiol or less) were not associated with higher breast cancer risks.

The findings of the study are in line with previous studies linking birth control pills and breast cancer. Estrogen is a powerful hormone that causes epithelial cells in the breasts to proliferate, resulting in breast growth. The risk of breast cancer returns to normal for women who stop using birth control pills.