Researchers from the Fred Hutchinson Cancer Researcher Center found that when women use depo-medroxyprogesterone acetate (DMPA), a progestin-only form of birth control, for a year or more, their breast cancer risk may be increased 2.2-fold. MSNBC reported that DMPA is also known as Depo-Provera.
Researchers noted that the increased risk seemed to go away just months after a woman stopped using the birth control.
"Our study adds to the body of knowledge from international studies conducted in a diverse group of countries -- Kenya, New Zealand, Thailand, Mexico and Costa Rica -- which have shown that one of the risks associated with DMPA use may be an increased risk of breast cancer," study researcher Dr. Christopher I. Li, M.D., Ph.D. said in a statement.
Li and his colleagues' study included 1,028 women ages 20 to 44 with breast cancer, and 919 people in the same age range with no breast cancer history. Ten percent of the women in the study reported using the DMPA birth control method.
MSNBC reported that about 3.2 percent of all women who use birth control use Depo-Provera, and those who do receive the shot every three months. FamilyDoctor.org reported that women shouldn't use Depo-Provera for a period longer than two years (unless there are no other options), since it can lead to calcium loss from bones.
HealthDay reported, though, that women should take the study findings with a grain of salt. The NYU Clinical Cancer Center's director of breast surgery, Dr. Freya Schnabel, told the publication that women in the study with the highest breast cancer risk who took the drug were also those with two big breast cancer risk factors: Having a family history, and never having had a child before.
"The study did not include information about all breast cancer risk factors in the participants, and this is a real limitation of the analysis which could impact on the results," Schnabel told HealthDay. "Also, the mechanism by which the progesterone would increase risk only in current users is not clear."
According to the National Cancer Institute, taking oral birth control is linked with a small breast cancer risk, as well as a lower endometrial and ovarian cancer risk.