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Making sense of associations between type 2 diabetes, metformin, and breast cancer risk


The relationship between type 2 diabetes (T2D), metformin, and breast cancer is complex. T2D may increase risk, but metformin used as first-line treatment of T2D may decrease breast cancer risk. This comment explores efforts to disentangle effects of T2D and metformin use on breast cancer risk in a prospective study.

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Authors and Affiliations



D.P.S was invited to write this piece by the British Journal of Cancer. Y.-M.M.P. and D.P.S. contributed equally to the writing of the Comment.

Corresponding author

Correspondence to Dale P. Sandler.

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Ethics approval and consent to participate

Not applicable. This comment includes published findings from the Sister Study. The Sister Study has been approved by the Institutional Review Board of the NIH. All participants provided written informed consent.

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Not applicable.

Data availability

Data from the Sister Study may be requested following procedures outlined at

Competing interests

The authors declare no competing interests.


This work was supported by the Intramural Research Program of the NIH, the National Institute of Environmental Health Sciences (Z01-ES044005 to DPS).

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Park, YM.M., Sandler, D.P. Making sense of associations between type 2 diabetes, metformin, and breast cancer risk. Br J Cancer 125, 909–910 (2021).

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