ethynilestradiol

Ethinylestradiol, also spelled ethinylestradiol, is a synthetic estrogen used primarily in hormonal contraception and in some forms of hormone replacement therapy. It is a derivative of estradiol in which an ethynyl group is attached at the 17α position, which increases oral potency and reduces first-pass metabolism. As an estrogen receptor agonist, it promotes estrogenic effects in target tissues and acts to suppress the hypothalamic-pituitary-ovarian axis when combined with a progestin in birth control pills.

Medical uses and administration vary by regimen. In combination oral contraceptives, ethinylestradiol is paired with a progestin to provide pregnancy prevention, menstrual regulation, and other cycle-related effects. It is also found in certain hormone replacement therapy regimens for menopausal symptoms in some countries, though dosing and formulations differ by product. Typical low-dose regimens use about 20–35 micrograms of ethinylestradiol per tablet, with a one-month cycle of daily tablets followed by a hormone-free interval or a continuous schedule, depending on the formulation.

Pharmacology and pharmacokinetics are characterized by high oral bioavailability compared with natural estradiol, extensive hepatic metabolism, and binding to plasma proteins such as albumin and sex hormone-binding globulin. It has a relatively long clinical duration and is primarily cleared through hepatic pathways.

Safety considerations are central to its use. Ethinylestradiol increases the risk of venous thromboembolism, particularly in smokers or older users, and is contraindicated in pregnancy, estrogen-sensitive cancers, significant liver disease, and undiagnosed uterine bleeding. Drug interactions include enzyme inducers that can reduce effectiveness, such as certain anticonvulsants and rifampin. Common side effects include nausea, breast tenderness, headaches, and mood changes.