Fulvestrant, sold under the brand name Faslodex among others, is an antiestrogenic medication used to treat hormone receptor (HR)-positive metastatic breast cancer in postmenopausal women with disease progression as well as HR-positive, HER2-negative advanced breast cancer in combination with abemaciclib or palbociclib in women with disease progression after endocrine therapy.

[6] It works by binding to the estrogen receptor and destabilizing it, causing the cell's normal protein degradation processes to destroy it.

[7] Fulvestrant is used for the treatment of hormone receptor positive metastatic breast cancer or locally advanced unresectable disease in postmenopausal women; it is given by injection.

[5] It is also used to treat ER-positive, HER2-negative advanced or metastatic breast cancer in combination with abemaciclib or palbociclib in women with disease progression after first-line endocrine therapy.

[2] Due to the medication's having a chemical structure similar to that of estrogen, it can interact with immunoassays for blood estradiol concentrations and show falsely elevated results.

[2][1] Very common (occurring in more than 10% of people) adverse effects include nausea, injection site reactions, weakness, and elevated transaminases.

Common (between 1% and 10%) adverse effects include urinary tract infections, hypersensitivity reactions, loss of appetite, headache, blood clots in veins, hot flushes, vomiting, diarrhea, elevated bilirubin, rashes, and back pain.

[1] Fulvestrant can form colloidal aggregates at certain concentration ranges and this can limit its activity as well as produce bell-shaped concentration–response curves.

[36] Because fulvestrant cannot be given orally, efforts have been made to develop SERD drugs that can be taken by mouth, including brilanestrant and elacestrant.

[6] The clinical success of fulvestrant also led to efforts to discover and develop a parallel drug class of selective androgen receptor degraders (SARDs).