I am not aware of any formal recommendations on this and there isn't much in the literature to guide us. Certainly, there is concern for Tamoxifen use in breastfeeding mothers as it's excretion into breastmilk is unknown and it is also known to suppress lactation. There is a trial, the POSITIVE (Pau...

If a patient is breastfeeding tamoxifen is not recommended since its excretion into breast milk is not known and that it has been shown to inhibit lactation. This is based on 2 published trials. Given the risk of venous thromboembolism (VTE) it should likely not be restarted during the postpartum pe...