Abstract: Objective Statins are the most common class of cholesterol-lowering oral medications, and the aim of this study was to genetically investigate the protective effects of such drugs against breast cancer. Methods Instrumental variables for the statin target gene HMGCR and five other cholesterol-regulated genes (i.e., LDLR, PCSK9, ABCG8, APOB, and NPC1L1) were extracted from previous expression quantitative trait locus (eQTL) studies; cholesterol-regulated genes predicted by these instrumental variables were used as the exposure factors to explore the effects of exposure factors on all breast cancers, estrogen, and breast cancer, by using the Mendelian randomization method based on pooled data ( SMR) was used to explore the genetic effects of exposure factors on the risk of all breast cancers, estrogen receptor-positive (ER+) breast cancers, and ER-breast cancers. Instrumental variables for total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and non-high-density lipoprotein cholesterol (non-HDL-C) were extracted from a previous human genome-wide association study (GWAS), and restricted to be chromosomally located within 100 kb of the above cholesterol regulatory genes, so that the instrumental variables could predict the cholesterol-associated genes mentioned above. TC, LDL-C, or nonHDL-C levels under regulation and used as exposure factors; two-sample Mendelian randomization (i.e., IVW, MR-PRESSO, and MR-Egger) was used to explore the genetic effects of exposure factors on the risk of all breast cancers, ER+ breast cancers, and ER-breast cancers. Results SMR analysis reported that elevated HMGCR expression was significantly associated with increased risk of all breast cancers and ER+ breast cancers (P = 0.044, P = 0.039), but not with change in risk of ER-breast cancers (P = 0.190); the other five regulatory genes did not significantly correlate with change in risk of all breast cancers, ER+ breast cancers, and ER-breast cancers (P > 0.05).IVW analysis reported that under the regulation of HMGCR, elevated levels of peripheral TC, LDL-C, and nonHDL-C significantly increased the risk of all breast cancers (P = 1.160e-05, P = 1.248e-05, P = 1.869e-05), as well as significantly increased the risk of ER+ breast cancer (P = 3.181e-04, P = 2.231e-04, P = 3.520e-04), but was not associated with a change in the risk of ER-breast cancer (P = 0.062, P = 0.133, P = 0.055).The results of MR-PRESSO and MR-Egger analyses supported the IVW results. Conclusion Statin use reduces the risk of ER+ breast cancer at the genetic level, whereas there is no such protection against ER- breast cancer. The above results contribute to an in-depth understanding of the role of cholesterol metabolism in the pathogenesis of breast cancer and provide a theoretical basis for the development of new anti-breast cancer therapeutic strategies.