Single-Drug and Combo Birth Control Pills Both Carry Increased Breast Cancer Risk: Study

Although progesterone-only birth control pills have long been recommended as a safer alternative to combination hormone pills, the findings of a new study suggests that both versions may increase the risk of breast cancer.

Researchers from the University of Oxford in the United Kingdom have released new data, which appear to refute decades of research pointing to single-hormone birth control as less likely to cause breast cancer when compared to dual-hormone birth control pills, like estrogen and progesterone combination pills. The findings were published on March 21 in the journal PLoS Medicine.

The study indicates the risk of breast cancer for current or former users of combination birth control pills that contain both synthetic versions of estrogen and progesterone face a small increased risk of developing breast cancer. A similar risk was also seen for users of progesterone-only contraception.

Researchers used data from the UK primary care database, the Clinical Practice Research Datalink, and compared it in a nested case-control study, involving 9,498 women younger than 50 with incident invasive breast cancer who were diagnosed between 1996 to 2017. Participants were matched with 18,171 control participants.

The researchers focused on all types of contraception, including single-hormone and dual-hormone birth control pills, single-hormone intrauterine devices (IUDs), injectable progesterone, and progesterone implants. These methods range from single and combination hormonal birth control pills to hormonal IUDs like Mirena and Skyla, hormonal injections like Depo-Provera, and hormonal implants like Nexplanon.

Birth Control Breast Cancer Risks

Overall, 44% of women with breast cancer and 39% of matched controls used a hormonal contraceptive prescription an average of three years before being diagnosed with breast cancer. Among prescriptions for birth control, half the prescriptions were for progesterone-only pills.

The findings indicated the risk of developing breast cancer is the same across all types of hormonal birth control options, including single hormone and combination hormone birth control pills.

The risk of developing breast cancer was increased by 20–30% among women who use combination hormone pills, progesterone-only pills, and hormonal IUDs, like Mirena or Skyla, which are progesterone-only IUDs, progesterone injections like DepoProvera, and progesterone implants like Norplant or Jadelle.

The findings indicate progesterone-only pills increase the risk of breast cancer similar to the risk of combination hormone pills. The increased risk with five years of use is estimated at eight additional cases of breast cancer per 100,000 users among ages 16 to 20 years old and 265 cases per 100,000 users among ages 35 to 39 years old

Research continues to focus on the risk of breast cancer linked to hormonal contraceptive use because breast cancer is hormone sensitive. Hormones can activate cancer cells and stimulate growth.

Roughly 65% of women in the U.S., or more than 72 million women, use some type of birth control, and one-quarter of those use hormonal birth control methods.

“This study provides important new evidence that current or recent use of progestogen-only contraceptives is associated with a slight increase in breast cancer risk, which does not appear to vary by mode of delivery, and is similar in magnitude to that associated with combined hormonal contraceptives,” researchers concluded.