Efficacy of Plan B. Plan B pills contain a high dose of the active ingredient Levonorgestrel, which is a second generation progestin. Progestins, just like natural Progesterone cause the uterine endometrium to slough thus causing a period. It works mainly by stopping the release of an egg from the ovary. It is possible that it may also work by preventing fertilization of an egg and its attachment to the uterus.