How effective is plan b in preventing pregnancy?

Plan B: Plan b taken after intercourse reduces the chance of pregnancy by 88%. In other words in prevents 7 out of 8 pregnancies that would have occurred.

It depends on when: it is taken. Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.