Plan B (often called emergency contraception) is an option after unprotected sex or contraceptive mishaps. Timing matters: it affects how well Plan B works, what side effects you might experience, and whether you should consider alternatives. This guide covers when to take Plan B, how timing changes its effectiveness, common side effects, and when to contact a healthcare provider so you can make informed choices.
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Take Plan B as quickly as possible after unprotected sex—speed matters. The usual recommendation is to take it within 72 hours, and it works best when taken in the first 72 hours . Waiting longer reduces the chance it will prevent pregnancy.
The standard window is within 72 hours after unprotected intercourse, with highest effectiveness if taken within the first 24 hours. Because Plan B mainly delays ovulation, earlier use gives the best chance of preventing pregnancy.
Research consistently highlights that emergency contraception is most effective the sooner it’s taken.
Plan B effectiveness window and declineEmergency contraceptives can be effective up to five days after unprotected sex, but peak effectiveness is much earlier. In some regions (sold as Plan B in the US/Canada and Levonelle in the UK), effectiveness is strongest within the first 12 hours and drops over time.Emergency Contraceptives–its Awareness, Practice, Impediments in Acceptance by Families of Defence Personnels at Cantonment.
Plan B is intended for use within 72 hours and becomes less reliable after that window. If more than 72 hours have passed, consider other emergency options—most notably the copper IUD, which can prevent pregnancy when inserted up to five days after unprotected sex.
Timing is a major factor: the sooner you take Plan B, the greater the likelihood it will prevent pregnancy.
Estimates suggest Plan B is about 95% effective within 24 hours, around 89% effective within 48 hours, and roughly 75% effective if taken within 72 hours. These figures also depend on where you are in your ovulation cycle at the time of exposure.
Clinical analyses show a clear decline in levonorgestrel emergency contraception’s effectiveness as more time passes after intercourse.
Levonorgestrel emergency contraception: effectiveness by timingWhen ovulation is fully prevented, levonorgestrel’s potential effectiveness ranged from about 90% with no delay to roughly 16% with a 72-hour delay. If ovulation was not fully inhibited, effectiveness estimates ranged from about 49% (no delay) to 8% (72-hour delay).Levonorgestrel emergency contraception: a joint analysis of effectiveness and mechanism of action, RT Mikolajczyk, 2007
Plan B mainly works by preventing or delaying ovulation. If the egg has already been released, Plan B may not stop fertilization. Taking it quickly increases the odds it will interrupt ovulation before the egg is released. You can learn more about birth control pills.
Expert reviews identify prevention or delay of ovulation as the primary way levonorgestrel-based emergency contraception works.
Plan B: mechanism of actionLevonorgestrel is believed to act primarily by preventing or delaying ovulation, and may also affect sperm or egg transport. Leading experts describe inhibition or delay of ovulation as the main mechanism behind levonorgestrel emergency contraception.Does levonorgestrel emergency contraceptive have a post-fertilization effect?A review of its mechanism of action, J Tudela, 2016
Plan B is generally safe, though it can cause temporary side effects. Knowing what to expect helps you plan and know when to seek care.
Common reactions include nausea, tiredness, headache, dizziness, breast tenderness, and changes in menstrual bleeding. These effects are usually mild and resolve within a few days.
Plan B can cause short-term changes to your period—your next period may come earlier or later than usual, and flow can be lighter or heavier. These shifts are typically temporary and should return to normal in subsequent cycles. If your period is 2-5 days late, it may be worth consulting a healthcare professional.
Contact a clinician if you have concerns about side effects, if you took Plan B outside the recommended timeframe, or if you’re unsure about next steps.
Seek medical care for severe abdominal pain, very heavy bleeding, or if your next period is more than a week late—these could signal complications or pregnancy and should be evaluated.
If Plan B is taken beyond its optimal window, the copper IUD is a highly effective alternative and can be used up to five days after unprotected sex. Talk with a healthcare provider to determine the best option for your situation.
If you vomit within two hours of taking Plan B, contact a provider or pharmacist about repeating the dose because your body may not have absorbed it. Vomiting after two hours usually means the pill was absorbed. If vomiting continues or you’re worried, seek medical advice.
No. Plan B is for emergency use and is not as reliable as regular contraception. It does not provide ongoing protection. For routine birth control, consider options like daily pills, IUDs, implants, or other methods—speak with a healthcare provider to find what fits you best.
No. Plan B prevents pregnancy after unprotected sex but does not protect against STIs. Use condoms to reduce STI risk, and get tested or talk with a provider if you have concerns. birth control methods are important to understand.
Plan B is emergency contraception used to prevent pregnancy before it occurs, primarily by delaying ovulation. The abortion pill (for example, mifepristone) is used to terminate an established pregnancy. They serve different purposes and are used at different stages.
Yes—Plan B can be taken if you’re using regular birth control and experience a failure (missed pill, broken condom, etc.). Continue your regular method as directed, and consult a provider if you’re unsure about interactions or next steps.
What should I do if I have unprotected sex again after taking Plan B?
If you have another incident of unprotected sex after taking Plan B, you may need another dose or a different emergency option depending on timing. Frequent need for emergency contraception is a sign to discuss a reliable, ongoing method with a healthcare professional.
Timing matters with Plan B: taking it as soon as possible gives you the best chance to prevent pregnancy, and being aware of side effects and alternatives helps you make safer choices. If you have questions, experience concerning symptoms, or need a more effective option after the 72-hour window, contact a healthcare provider for personalized guidance. Learning more about emergency contraception can help you feel confident in your reproductive health decisions.
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