Plan B One-Step is a type of emergency contraception. This is birth control that can prevent pregnancy after unprotected sex. People sometimes call it the "morning after pill." But you don't have to wait until the morning after sex to take it. In fact, Plan B is more effective the sooner you take it. It is a one-dose regimen: you take one pill. The pill contains 1.5 milligrams of levonorgestrel, which is used in lower doses in many birth control pills.