Although there's some evidence to suggest that statins reduce the threat of breast cancer, a large, prospective cohort study has concluded that serum lipid levels and cholesterol-lowering drugs, including statins, do not seem to affect a woman's risk of breast cancer.  

Researchers followed almost 80,000 cancer-free women between the ages of 42 and 69 who participated in the Nurses' Health Study for up to 12 years. A total of 3,177 cases of invasive breast cancer were documented. The researchers found that current users of lipid-lowering drugs in general and users of statins in particular had a breast cancer risk similar to nonusers (multivariate RR, 0.99; 95% CI; 0.86–1.13 and RR 0.91; 95% CI; 0.76–1.08, respectively). They also found that women with self-reported serum cholesterol levels of  ≥240 mg/dL were at no greater risk for breast cancer than women with levels <180 mg/dL (RR 1.04; 95% CI; 0.91–1.17). Duration of use was similarly unassociated with risk.  

Eliassen AH, Colditz GA, Rosner B, et al. Serum lipids, lipid-lowering drugs, and the risk of breast cancer. 

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Although there's some evidence to suggest that statins reduce the threat of breast cancer, a large, prospective cohort study has concluded that serum lipid levels and cholesterol-lowering drugs, including statins, do not seem to affect a woman's risk of breast cancer.