Plan B is most effective within the first 24 hours after unprotected sex, when it prevents pregnancy about 94% of the time. Its effectiveness drops significantly over the next two days, falling to roughly 58% by the 72-hour mark. It does not provide any ongoing protection against pregnancy from future sexual encounters.
The 72-Hour Window
Plan B is designed to be taken within 72 hours (three days) of unprotected sex, but the clock matters enormously. At 24 hours, it’s about 94% effective. By 72 hours, that number drops to around 58%. The CDC notes that emergency contraception pills can technically be taken up to five days (120 hours) after unprotected sex, but levonorgestrel (the active ingredient in Plan B) works best in the first three days. If you’re closer to the five-day mark, a different type of emergency contraception, the copper IUD or a prescription pill called ella, is a more reliable option.
The simple rule: the sooner you take it, the better it works. Every hour you wait reduces its ability to do its job.
How Plan B Actually Works
Plan B works by delaying ovulation, the release of an egg from the ovary. If there’s no egg available, sperm can’t fertilize anything. This is why timing matters so much. If your body has already begun the hormonal surge that triggers ovulation, Plan B loses its ability to stop or delay that process. Research published in the journal Contraception found that women who took levonorgestrel at or after this hormonal surge had ovulation and conception rates similar to women who took nothing at all.
This means Plan B is most useful in the earlier part of your cycle, before ovulation has kicked off. It does not prevent a fertilized egg from implanting, and it will not end an existing pregnancy.
It Does Not Protect You After You Take It
A common misconception is that Plan B offers some window of ongoing protection, similar to how a regular birth control pill works. It doesn’t. Because it works solely by delaying ovulation, a single dose only addresses the risk from the specific encounter that prompted you to take it. If you have unprotected sex again later in the same cycle, you’d need to take it again for it to be effective. Plan B is not a substitute for regular contraception.
Effects on Your Next Period
Plan B will likely shift the timing of your next period. Most people find their period arrives within a week of the expected date, either earlier or later than usual. Some spotting between periods is also common. If your period is more than a week late, that’s worth a pregnancy test, since it could mean the pill didn’t work.
Other temporary side effects can include nausea, headache, fatigue, and breast tenderness. These typically resolve within a day or two.
Weight Can Affect How Well It Works
Plan B becomes less effective at higher body weights. According to Planned Parenthood, if you weigh more than 165 pounds, levonorgestrel-based emergency contraception won’t work as well. The pill isn’t completely ineffective above that weight, but the reduction in efficacy is significant enough that you may want to discuss alternatives with a pharmacist or provider. The copper IUD, which can be placed up to five days after unprotected sex, is the most effective emergency contraception option regardless of weight.
Shelf Life if You Keep One on Hand
Plan B is available over the counter without a prescription or age restriction, and some people keep a box at home for emergencies. That’s a reasonable strategy, but check the expiration date before you need it. The pill can sit in your medicine cabinet for a few years, but expired Plan B loses potency. An expired pill is not considered reliable, and given how time-sensitive the situation already is, starting with a compromised dose isn’t ideal. The expiration date is printed on the side of the box.