Answer
Breastfeeding is not recommended because tamoxifen and active metabolites can accumulate in milk and tamoxifen can suppress lactation.
Sources for this answer
- Tamoxifen 20 mg Tabletsemc · SmPC Current selected SmPC · Version 7
- Tamoxifen Rosemont 10mg/5ml Oral Solutionemc · SmPC Current selected SmPC · Version 17