Tamoxifen, a drug used to reduce the risk of breast cancer occurring in breast cancer survivors may actually increase the risk of an aggressive form of breast cancer in women who use it over the long term, according to a new study. The study, published in the Journal of Cancer Research looked at women whose breast cancer tumors were estrogen sensitive.

While the study found that women who took Tamoxifen were 60 percent less likely to develop a new estrogen-sensitive type of breast cancer, long term users were 40 percent more likely to develop a rare, aggressive form of breast cancer that is not sensitive to estrogen.

According to the study's lead author women should still take the drug, as the benefits outweigh the risks. The drug is usually only prescribed for 5 years, because there is no evidence of benefits continuing beyond that timeframe.