Estradiol cypionate has not been studied during breastfeeding. However, a similar drug, estradiol valerate, has been used to suppress lactation, usually in combination with testosterone. Generally, estradiol cypionate should be avoided in mothers wishing to breastfeed, especially if started before the milk supply is well established at about 6 weeks postpartum. The decrease in milk supply can happen over the first few days of estrogen exposure.[1]