Pregnancy doesn't happen right after sex. That's why it's possible to prevent pregnancy even after the fact. It can take up to six days for the sperm and egg to meet after having sex. Emergency contraception pills work by keeping a woman's ovary from releasing an egg for longer than usual. Pregnancy cannot happen if there is no egg to join with sperm.
Plan B One-Step and Next Choice One Dose, are up to 89 percent effective when taken within 72 hours (three days) after unprotected sex. They continue to reduce the risk of pregnancy up to 120 hours (five days) after unprotected sex, but they are less effective as time passes.
Side effects are uncommon of Plan B, and usually stop within a day or two. Possible side effects include an earlier or later, heavier or lighter period than usual. Nausea or vomiting. If you vomit within two hours of taking the pill(s), it won't be effective and you need to take it again.
Frequent use may cause periods to become irregular and unpredictable. Emergency contraception should not be used as a form of ongoing birth control because there are other forms of birth control that are a lot more effective and less expensive.
Anyone — no matter how old you are — can get Plan B One-Step over the counter without a prescription at a drugstore, Planned Parenthood health center, or family planning clinic.
You can read more about it here: http://www.planbonestep.com/
Have you ever taken Plan B? Did it work for you? Any side effects?