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You can take plan b again.
Plan B is effective if taken within 72 hours of intercourse. It is most effective when taken within 24 hours after unprotected intercourse. It does not matter what the half life is, or how long it is in your system. It is the timing that is important. You can take another Plan B. It is likely that the one you took Sunday is already working but it is not harmful to take another. You need to use condoms the rest of the month as you will not want to keep taking Plan B. When you get your period, be sure to start a reliable form of birth control
The adverse effects mainly center around abnormal bleeding and irregular bleeding. You cannot expect the cycle to to be normal. It is ok to take it in the first 72 hours after unprotected sex but it is most effective if taken within 24 hours. So it would be better to take it tonight.