Oral contraception is the use of birth control pills or oral contraceptives in preventing unwanted pregnancy. Women who use this method have to take a pill every day at around the same time of day. Most pills contain estrogen and progestin, female sex hormones which prevent ovulation, which make the uterine lining thinner to prevent the progress of pregnancy, and thicken cervical mucus to block sperm. It is important to note that first-time users need to use back-up birth control for 7 days (i.e. condoms) while the drug has not yet fully taken effect. When used properly, the effectivity of oral contraception is 99.9%. The side effects may include nausea, bloating, breast tenderness, vaginal spotting, and abnormal bleeding. Oral contraceptives may also be prescribed to treat endometriosis, premenstrual syndromes, polycystic ovarian syndrome, acne, irregular periods, and painful periods.