Ethinyl estradiol is a synthetic form of the hormone estrogen, commonly used in various hormonal contraceptives, including birth control pills. It helps regulate the menstrual cycle and can prevent ovulation, making it effective for contraception. In addition to its use in birth control, ethinyl estradiol is also prescribed to treat certain medical conditions, such as acne and polycystic ovary syndrome (PCOS). It works by balancing hormone levels in the body, which can alleviate symptoms associated with these conditions.