The Plan B/ the"morning after" pill/emergency contraception contains the hormone levonorgestrel (a progestin) which can stop the person taking it from ovulating, block fertilization and/or keep a fertilized egg from implanting in the uterus. Plan B must be taken within 72 hours of unprotected sex or contraceptive failure - including condom breakage - but is more effective the sooner it is taken.
A pregnancy test is the only way to confirm whether or not Plan B has been effective, and should be taken after the first day of a missed period.