Does plan b work?

Yes: Plan b is about 90% effective if taken within 3 days (72 hours) of having unprotected sex. It may still work up to 5 days (120 hours) after unprotected sex, but the failure rate is higher.

Plan B can work: 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.