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Birth control shots tied to breast cancer risk, study says

Recent use of the injectable contraceptive Depo-Provera for at least a year was associated with a doubling of young women’s breast cancer risk, a new study has found.

However, users’ breast cancer risk dropped to that of non-users within several months of stopping Depo-Provera injections, researchers report in the journal Cancer Research.

Depo-Provera, injected every three months, was approved as a contraceptive in the United States 20 years ago. Convenient, highly effective and relatively inexpensive, Depo-Provera is used by about 1.2 million U.S. women, or 3.2 percent of those who practice contraception, according to the latest data from the Guttmacher Institute, a research and education organization that focuses on reproductive health. 

The injectable birth control method is the only contraceptive in the United States that contains the same progestin, or synthetic hormone, as Prempro, the postmenopausal hormone therapy pill. A landmark government study called the Women’s Health Initiative found that Prempro, a combination of estrogen and progestin, increased women’s breast cancer risk by 24 percent, while Premarin, which contains only estrogen, did not increase risk.  

‘’Our hypothesis going into this study was that we did expect to see an increased risk of breast cancer associated with Depo-Provera,” says Dr. Christopher Li, a breast cancer epidemiologist at the Fred Hutchinson Cancer Research Center and lead author of the new study.

Data on the relationship between Depo-Provera and breast cancer risk are limited, the researchers write. Li and his coauthors say theirs is the first large-scale U.S. study specifically designed to evaluate the relationship. Results from similar studies conducted in other countries have been mixed, they write.

Li’s team recruited 1,028 women who’d been diagnosed with breast cancer and 919 women who had not. All the women were 20 to 44 years of age and lived in the Seattle area. About 3 percent had used Depo-Provera within the last five years.

Compared to women who had never used Depo-Provera, those who had received injections within the previous five years were 2.2 times more likely to have been diagnosed with breast cancer, the scientists found. Family history, obesity, age and pregnancy history didn’t seem to make a difference.

Age is the main risk factor for breast cancer, so while a doubling of risk might sound alarming, Li emphasizes that the actual number of breast cancer cases in women in their 20s and 30s is very low.

“Breast cancer among young women is still a rare disease,” he says. According to the National Cancer Institute, women in their 30s have a 1 in 233 chance of being diagnosed with the disease. By comparison, the odds of being diagnosed with breast cancer for women in their 60s is 1 in 29.

“However,” Li and his coauthors write, “there are numerous contraceptive options, and so further clarifying the benefits and risks associated with each option is important as women consider what choices might be best for them.”

Joan Campion, spokewoman of Pfizer, the maker of Depo-Provera, said, "As part of the Depo-Provera label, physicians are already advised on benefits and risks of Depo-Provera, including the risk of breast cancer. Pfizer currently believes that changes to benefits and risk profile are not warranted as a result of this observational study."