Reference summary — not medical advice or a personal risk estimate.
Not breastfeeding
Quick answer
Not IARC-classified — this is not a chemical or physical agent IARC rates. Recognized as a cancer risk factor based on observational evidence. Reproductive history. Applies to people who did not breastfeed, or breastfed for a shorter cumulative duration.
Summary
Breastfeeding for a year or more (cumulatively, across pregnancies) is associated with modestly lower breast cancer risk compared with not breastfeeding.
What's not established
This reflects an average association across study populations, not a guarantee for any individual, and the size of the protective association varies across studies and breast cancer subtypes. It is not a recommendation on infant feeding, which involves many factors beyond cancer risk.
Evidence sources
Every claim on this page traces back to one of these. Click through to read the primary document yourself.
Primary sources
NCI
NCI — Reproductive History and Cancer Risk
Last reviewed: October 2026
Official reference list
A government list some readers use alongside the primary evidence. Meyron is not affiliated with it. A chemical being listed does not by itself describe a risk from any particular product.
- California Prop 65 list
State of California's list of chemicals known to cause cancer or reproductive harm.