Objective  To explore the causal relationship between bipolar disorder and breast cancer and its subtypes (estrogen receptor-positive and estrogen receptor-negative breast cancer).

Methods  Using genome-wide association data of bipolar disorder and breast cancer, a two-sample Mendelian randomization analysis was performed to identify single nucleotide polymorphisms associated with bipolar disorder and explore their association with breast cancer. The inverse-variance weighting (IVW) method served as the primary research approach, while MR Egger, weighted median estimator, and weighted mode were employed as supplementary methods. Sensitivity analyses were performed to assess heterogeneity and pleiotropy.

Results  The IVW analysis revealed no causal relationship between bipolar disorder and overall breast cancer [OR=1.04, 95%CI(0.98, 1.12)], nor with its two subtypes (P>0.05). The supplementary methods were consistent with the IVW findings. Sensitivity analyses indicated that the results were reliable.

Conclusion  There is no causal relationship between bipolar disorder and the risk of developing overall breast cancer, nor is there support for a causal relationship with a specific subtype of breast cancer.

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