Numerous studies have suggested that birth control pills increase the risk of breast cancer. Now, a new study suggests that this increased risk may only apply to recent users and is dependent on the formulation of the pill.

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Researchers found that among recent users of birth control pills, breast cancer risk increased with only certain pill formulations.

The research team, including Elisabeth F. Beaber of the Fred Hutchinson Cancer Research Center in Seattle, WA, recently published their findings in the journal Cancer Research.

In the US, two types of birth control pills - also known as oral contraceptives - are currently available. The most common is what is known as the "combined" oral contraceptive. This consists of man-made interpretations of the female hormones estrogen and progesterone. The other oral contraceptive, known as the minipill, only contains progestin - a man-made version of progesterone.

It is well established that estrogen and progesterone that occur naturally in the body can increase the risk of some cancers. Some studies have suggested that man-made versions of the hormones found in oral contraceptives can have the same effect - particularly when it comes to breast cancer.

In this latest study, the team set out to see whether increased breast cancer risk could be attributed to certain types of man-made female hormones.

"Oral contraceptive formulations have changed over time in the US, and contemporary formulations have received relatively less scrutiny with respect to breast cancer risk," Beaber told us.

Recent use of certain oral contraceptives increased breast cancer risk by 50%

To reach their findings, Beaber and colleagues analyzed the use of oral contraceptive pills among 1,102 women who had been diagnosed with breast cancer between 1990 and 2009, alongside 21,952 healthy controls.

The team notes that unlike many previous studies looking at the link between birth control pills and risk of breast cancer, theirs gathered data on patients' use of the pills from electronic pharmacy records.

"The majority of prior studies in the US have relied on self-reported oral contraceptive formulation information, which may be less reliable than pharmacy data," Beaber told Medical News Today.

The results revealed that women who were recent users of oral contraceptives - defined as completing at least one prescription for the pills in the past year - had a 50% increased risk of breast cancer, compared with former users and those who had never used them.

But the team found that certain pills had a stronger effect than others. Oral contraceptives containing high-dose estrogen increased breast cancer risk 2.7-fold, while those containing moderate-dose estrogen presented a 1.6-fold increased risk. Oral contraceptives containing low-dose estrogen were found to have no effect on breast cancer risk.

Birth control pills containing ethynodiol diacetate - a form of progestin - increased breast cancer risk 2.6-fold. Triphasic combination pills (that consist of three different doses of hormones in every pack) containing 0.75 milligrams of norethindrone - another type of progestin - increased breast cancer risk by 3.1-fold.

Commenting on the team's findings, Beaber said:

"In our study, we found some variation in the risk of breast cancer associated with recent use of oral contraceptives. While some oral contraceptive formulations were associated with an increased breast cancer risk, others did not appear to be associated with an increased risk."

However, she stresses that the results should be interpreted with caution. "Given that these results have not yet been replicated and the importance of assessing both the benefits and risks of oral contraceptive use, we cannot make any clinical recommendations based on results from this single study."

She notes that breast cancer is rare among young women, and there are many well-established health benefits linked to oral contraceptive use that need to be considered. "In addition," she says, "prior studies suggest that the increased risk associated with recent oral contraceptive use declines after stopping oral contraceptives."

Furthermore, some studies claim that birth control pills may even reduce the risk of certain cancers. Earlier this year, Medical News Today reported on a study published in the Journal of the National Cancer Institute, revealing that oral contraceptives - as well as breastfeeding and tubal ligation - could reduce the risk of ovarian cancer for women with BRCA1 or BRCA2 gene mutations.