What is the difference between the plan b emergency contraceptive and regular birth control pills?

Mechanisms differ: Plan b is levonorgestrel. It alters the tubes to prevent passage of the eggs and sperm and prevent fertilization. It also alters the endometrium to prevent implantation. Birth control pills usually contain an estrogen and a progesterone. They suppress LH and FSH to prevent ovulation. They also alter the cervical mucous to prevent sperm from entering.

Not the same: Emergency contraception acts mainly by preventing ovulation, and is intended for only occasional or one-time use. Regular birth control pills also work by preventing ovulation, but they work on an ongoing basis for regular contraception (and require daily use). If you need emergency contraception, it's good to use it, but you should then consider ongoing birth control. See http://ec.Princeton.