Levonelle One Step is the branded version of Generic Levonorgestrel. Both contain the same active ingredient Levonorgestrel. The strength for the emergency contraception is 1500mg and it works by preventing the release of an egg and hence pregnancy. It does not completely guarantee that it will stop you from becoming pregnant, it should not be used as a substitute for regular contraception.
It is 84% effective if taken within 72 hours of having unprotected sex but can be up to 95% effective if taken within 1 day of intercourse. The sooner it is taken, the more effective it is.
Levonorgestrel will not protect you from sexually transmitted infections (STIs).