Plan B is most effective within the first 24 hours after unprotected sex, and its effectiveness drops steadily over the next two to three days. The official window is 72 hours (three days), but there is limited evidence it may offer some protection up to 120 hours (five days). The sooner you take it, the better it works.
Effectiveness by Time Window
Plan B works by delivering a high dose of a synthetic hormone that delays or prevents ovulation. If your body hasn’t released an egg yet, the pill can pause that process long enough for sperm to die off before fertilization can happen. It does not end an existing pregnancy.
The critical detail is that this mechanism becomes less reliable with every hour that passes. Within the first 24 hours, Plan B prevents the highest percentage of expected pregnancies. Between 24 and 48 hours, effectiveness drops noticeably. Between 48 and 72 hours, it drops further still. After 72 hours, Plan B is not officially recommended, though the World Health Organization notes that levonorgestrel-based pills can be taken up to 120 hours after unprotected sex. If you’re past the 72-hour mark, a different type of emergency contraceptive pill containing ulipristal acetate (sold as ella) is significantly more effective in that 72-to-120-hour window.
Why Timing Matters So Much
Plan B’s entire job is to stop ovulation before it happens. Once an egg has already been released, the pill has little to no effect. This is why the clock starts ticking immediately after unprotected sex. Sperm can survive inside the body for up to five days, so the longer you wait, the greater the chance that ovulation occurs before the drug can intervene. Taking Plan B within a few hours of unprotected sex gives the hormone the best chance of suppressing the hormonal surge that triggers egg release.
Body Weight Can Reduce Effectiveness
Research has found that Plan B becomes less reliable for people with a BMI above 26. The hormone in Plan B is sensitive to body weight, meaning higher BMI can reduce the drug’s concentration in the bloodstream enough to matter. If your BMI is above that threshold, a copper IUD inserted as emergency contraception is the most effective option regardless of weight. The ulipristal acetate pill (ella) also performs better than Plan B at higher body weights, though it still has some limitations.
Medications That Interfere With Plan B
Certain medications can make Plan B less effective by speeding up how quickly your body breaks down the hormone. St. John’s wort, a common herbal supplement, is one of the most well-known culprits. Several prescription medications, including some anti-seizure drugs, certain HIV treatments, and some antibiotics, can also reduce its potency. If you take any prescription medication regularly, it’s worth checking whether it interacts with levonorgestrel before relying on Plan B as your emergency option.
Using Plan B More Than Once
Plan B can be used more than once in the same menstrual cycle if needed. The American College of Obstetricians and Gynecologists supports repeated use of levonorgestrel emergency contraception when necessary, and studies have not found an increased risk of serious side effects from taking it multiple times. That said, repeated use in the same cycle can cause more menstrual irregularities, and Plan B is not designed to replace regular contraception. It is less effective than most ongoing birth control methods and more likely to cause cycle disruptions when used frequently.
Side Effects and What to Expect After
Most side effects from Plan B are mild and resolve within a few days. The most common ones include nausea, fatigue, headache, breast tenderness, and cramping. Some people experience light spotting between periods or heavier bleeding when their period does arrive.
Your next period may come earlier or later than expected. This shift is normal and results from the hormone temporarily disrupting your cycle’s usual timing. If your period is more than a week late, a pregnancy test is a reasonable next step, since no emergency contraceptive is 100% effective.
Shelf Life Before You Need It
If you keep Plan B on hand for emergencies, check the expiration date printed on the side of the box. The pill can last in your medicine cabinet for years, but an expired pill is less effective. Heat and improper storage can also degrade the medication over time. Replacing it before it expires ensures it will work at full strength when timing matters most.