It works mainly by stopping or delaying the ovaries from releasing an egg. It may also work by changing the lining of the womb that may prevent implantation of a fertilized egg.

Emergency contraceptives are most effective when taken within 12-24 hours after unprotected sex.
Depending on the type of emergency pill, it can be taken within up to 3 or 5 days after having unprotected sex and prevents pregnancy by releasing a (high) dose of hormones. For the best chance for it to work, you should take it as soon as possible.

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