The birth control pill is the most popular form of reversible birth control, with over 18 million US women relying on it. Oral contraceptives contain synthetic hormones that prevent pregnancy by stopping ovulation and making the uterus inhospitable for implantation. There are combination pills containing estrogen and progestin, as well as progestin-only pills. While generally safe and effective, birth control pills can have side effects like nausea, mood changes, and weight gain, as well as serious risks like blood clots and stroke. However, they also provide benefits like reducing cancer risks and improving symptoms of menstruation. Women's experiences with and opinions of birth control pills vary significantly.