You can take Plan B up to 72 hours (three days) after unprotected sex, but it works best the sooner you take it. Within the first 24 hours, Plan B prevents about 95% of pregnancies. After that, effectiveness drops steadily with each passing day.
How Effectiveness Changes Over Time
Plan B is most effective when taken as quickly as possible. In the first 24 hours, it prevents roughly 95% of pregnancies that would have otherwise occurred. Between 24 and 48 hours, effectiveness drops noticeably, and by the 48 to 72 hour window, it drops further still. The manufacturer lists 72 hours as the outer limit.
There is no benefit to waiting. If you know you need emergency contraception, taking it immediately gives you the best chance of preventing pregnancy. Even a few hours can make a difference, so treat it as a “right now” decision rather than a “morning after” one.
How Plan B Actually Works
Plan B works by delaying or preventing ovulation, the release of an egg from the ovary. Your body sends a hormonal signal (called an LH surge) that triggers ovulation. Plan B interrupts that signal, pushing ovulation back by several days so sperm already in your body die off before an egg is available.
This is important to understand because it means Plan B is most effective when taken before ovulation happens. If you’ve already ovulated, the pill has little to no effect. A 2022 review in the journal Contraception confirmed that women who took Plan B at or after ovulation had conception rates similar to women who took nothing at all. This is why timing matters so much: you’re racing against your own cycle, not just a clock.
When Plan B May Not Work Well
Body weight affects how well Plan B works. If you weigh more than 165 pounds, Plan B’s effectiveness drops significantly. This doesn’t mean it does nothing, but the hormonal dose may not be sufficient to reliably delay ovulation. If you weigh over 165 pounds, a prescription alternative called ella or a copper IUD are more reliable options (more on those below).
Certain medications also interfere with Plan B. Drugs that speed up your liver’s processing of hormones can cut the active ingredient’s blood levels dramatically. Some epilepsy medications, tuberculosis drugs, HIV treatments like efavirenz (which reduces levels by about 50%), and the herbal supplement St. John’s wort all fall into this category. If you’ve taken any of these within the past four weeks, Plan B may be less effective for you.
Alternatives With a Longer Window
If more than 72 hours have passed, Plan B is no longer a reliable option, but you still have two good ones. Ella, a prescription emergency contraceptive pill, works up to 120 hours (five days) after sex and maintains its effectiveness throughout that full window rather than declining sharply over time. It also works better than Plan B for people over 165 pounds, though its effectiveness decreases above 195 pounds.
The most effective option at any point in the five-day window is an IUD. The Paragard copper IUD, Mirena, and Liletta all work as emergency contraception when inserted within 120 hours of unprotected sex. They work just as well on day five as on day one, and they’re the single most effective form of emergency contraception available. As a bonus, they then serve as long-term birth control. Getting one requires a healthcare visit, but many clinics offer urgent appointments for this purpose.
What to Expect After Taking It
Plan B can cause nausea, fatigue, headache, and breast tenderness. These side effects are typically mild and resolve within a day or two. If you vomit within three hours of taking the pill, you may need to take another dose, so contact a healthcare provider if that happens.
Your next period may arrive earlier or later than expected. Some people see spotting in the days after taking Plan B, which is normal and not a sign that it failed. If your period is more than a week late, take a pregnancy test. A normal or even heavy period arriving on schedule is the most common outcome and a good sign the pill worked.
Plan B is safe for breastfeeding parents. The active ingredient passes into breast milk in minimal quantities, and studies have found no difference in breastfeeding outcomes between people who took it and those who didn’t.
Can You Take Plan B More Than Once?
Plan B is safe to use more than once, even within the same menstrual cycle if needed. It is not an abortion pill and won’t affect an existing pregnancy. However, it’s less effective than regular contraception, and repeated use can make your periods unpredictable. If you find yourself reaching for Plan B frequently, a daily pill, IUD, implant, or other method will give you far more reliable protection with fewer hormonal disruptions.