A study conducted on 1.8 million women for more than a decade has found that women using birth control pills and I.U.D.’s that release hormones have a significantly increased risk of developing breast cancer than women that do not hormones. The researchers concluded that hormone users experienced a 20% increase in the relative risk of breast cancer compared to non-users, and that the risk increased with age and length of use.

In fact, they found that women that used hormones for five years or more had an increased risk of breast cancer even after they discontinued use. Women that stayed on hormone for ten years or more experienced a 38% increase in their relative risk of developing breast cancer compared to non-users. The good news is that the study found that there was no increased risk for breast cancer seen in women who used hormones for less than one year.

The link between hormonal birth control and breast cancer was established over 20 years ago; however, this study was the first to examine the risks associated with newer hormone-based formulations in a large population. The study found little difference in breast cancer risk amongst the formulations, and found that progestin – a synthetic form of progesterone – may be one of causes of increased breast cancer risk.