Final answer:
Tamoxifen is the drug used to treat estrogen receptor-positive breast cancers. It is a selective estrogen receptor modulator (SERM) that blocks the growth-promoting effects of estrogen on breast cancer cells.
Step-by-step explanation:
The drug used to treat estrogen receptor-positive breast cancers is Tamoxifen. Tamoxifen is a selective estrogen receptor modulator (SERM) that binds to the estrogen receptor and prevents it from activating genes, thus blocking the growth-promoting effects of estrogen on breast cancer cells. It is commonly used as adjuvant therapy to reduce the risk of recurrence in women with hormone receptor-positive breast cancer.