The active ingredients in morning-after pills are similar to those in birth control pills, except in higher doses. Some morning-after pills contain only one hormone,
levonorgestrel (Plan B, Plan B One-Step), and others contain two, progestin and estrogen. Progestin prevents the sperm from reaching the egg and keeps a fertilized egg from attaching to the wall of the uterus (implantation). Estrogen stops the ovaries from releasing eggs (ovulation) that can be fertilized by sperm.