Using progestin-only birth control shots or Depo-Provera for more than a year appears to more than double the risk of developing breast cancer in young women, according to a new study.

The study compared 1,028 women between the ages of 20 to 44 in Seattle who had been diagnosed with breast cancer to 919 other matched participants without breast cancer and found that women who used depo-medroxyprogesterone acetate, or DMPA, a contraceptive containing the same kind of progestin found in menopausal hormone-therapy regimen, had a 2.2-fold increase risk of developing invasive breast cancer. 

“While DMPA is widely used by women throughout the world, there are limited data on the association between DMPA and breast cancer incidence,” lead investigator Dr. Christopher Li, a breast cancer epidemiologist at the Fred Hutchinson Cancer Research Center in Seattle, said in a statement released on Wednesday from the research center.. “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,” he said.

Researchers found that the increased risk for breast cancer seemed to fade months after women stopped using the contraceptive.  They also found that women who used the contraceptive for less than a year or those who had stopped using it for more than a year did not have an increased risk for the disease.

"Although breast cancer is rare among young women and the elevated risk of breast cancer associated with DMPA appears to dissipate after discontinuation of use, our findings emphasize the importance of identifying the potential risks associated with specific forms of contraceptives given the number of available alternatives," researchers wrote in the study.

Of all the participants in the study, about 10 percent reported using DMPA, consistent with usage patterns nationwide, according to Li, and he noted that the use of injectable contraceptive was about 5 percent higher among non-white women in the study.

“In the United States, many women have numerous options for contraception, and so it is important to balance their risks and benefits when making contraceptive choices," Li said.

"This study further confirms that some types of progestins are not healthy for the breast. For women who are at elevated risk to develop breast cancer based on family history, or even age, this type of contraception may not be a good choice for them," Dr. Elizabeth Poynor, a gynecologic oncologist and pelvic surgeon at Lenox Hill Hospital in New York City, said commenting on the study, according to HealthDay.

While the latest study found a link between Depo-Provera and elevated breast cancer risk, it did not prove that injectable contraceptive caused the increased risk of disease. 

Dr. Freya Schnabel, director of breast surgery at NYU Clinical Cancer Center in New York City, said that the women in the study who seemed to be at highest risk of developing breast cancer while using Depo-Provera had other risks factors linked to the disease, like having a family history of breast cancer or they have never had children, according to HealthDay.

She also said that the study had limitations because it did not include all the breast cancer risk factors in the women who participated in the study, and because researchers did not explain “the mechanism by which the progesterone would increase risk only in current users”.

The study is published online and in the April 15 print issue of the journal Cancer Research.