Fulvestrant, sold under the brand name Faslodex among others, is a 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 palbociclib in women with disease progression after endocrine therapy. It is given by injection into a muscle. Fulvestrant is a selective estrogen receptor degrader (SERD) and was first-in-class to be approved. It works by binding to the estrogen receptor and destabilizing it, causing the cell's normal protein degradation processes to destroy it. Fulvestrant was approved for medical use in the United States in 2002. Medical uses Breast cancer 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. A 2017 Cochrane review found it is as safe and effective as first line or second line endocrine therapy. It is also used to treat ER-positive, HER2-negative advanced or metastatic breast cancer in combination with palbociclib in women with disease progression after first-line endocrine therapy. 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. This can improperly lead to discontinuing the treatment. Early puberty Fulvestrant has been used in the treatment of peripheral precocious puberty in girls with McCune–Albright syndrome. Available forms Fulvestrant is provided in a castor oil solution also containing alcohol, benzyl alcohol, and benzyl benzoate. It is supplied at a concentration of 250 mg/5 mL. Contraindications Fulvestrant should not be used in women with kidney failure or who are pregnant. Side effects 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. In a large clinical trial, the incidence of venous thromboembolism (VTE) with fulvestrant was 0.9%. Pharmacology Pharmacodynamics Fulvestrant is an antiestrogen which acts as an antagonist of the estrogen receptor (ER) and additionally as a selective estrogen receptor degrader (SERD). It works by binding to the estrogen receptor and making it more hydrophobic, which makes the receptor unstable and misfold, which in turn leads normal processes inside the cell to degrade it. In addition