Plan B has a shelf life of about four years from the date of manufacture, and the expiration date is printed on the side of the box. But if you’re asking how long you have to take it after unprotected sex, the answer is up to 72 hours for best results, with some effectiveness extending to 120 hours (five days).
Most people searching this question mean one of those two things, so let’s cover both clearly.
How Long Plan B Lasts on the Shelf
Plan B can sit in your medicine cabinet for years before it expires. The exact expiration date, listed by month and year, is printed on the side of the retail box. Like all medications, the active ingredient gradually breaks down over time, which means an expired pill may not deliver its full dose.
To maintain potency, store Plan B at room temperature, between 68°F and 77°F. Leaving it in a hot car, a humid bathroom, or anywhere with temperature extremes can speed up degradation. If you’re keeping one on hand for emergencies, a bedroom drawer or closet shelf is a better choice than a glove compartment.
If the only pill you have access to is expired, taking it is still reasonable. An expired pill is unlikely to be harmful. It just may be less effective than a fresh one. In a time-sensitive situation, a slightly degraded dose is better than no dose at all.
How Long You Have to Take It After Sex
Plan B works best when taken as soon as possible after unprotected sex. Within the first 24 hours, it’s most effective. It still works well within 72 hours (three days), and it retains some effectiveness out to 120 hours (five days), though results drop off as time passes.
Studies show that levonorgestrel emergency contraception has a pregnancy rate of roughly 1.2% to 2.1% when taken within the recommended window. That rate climbs the longer you wait. Every hour counts, so if you know you need it, don’t wait until the next day just because you’re technically still within the window.
How Plan B Works
Plan B contains a single 1.5 mg dose of levonorgestrel, a synthetic hormone that primarily works by delaying or preventing ovulation. If your body hasn’t released an egg yet, the pill can postpone that release long enough for sperm to die off (sperm survive about five days in the reproductive tract). If ovulation has already happened, the pill is much less likely to prevent pregnancy. This is the main reason timing matters so much.
Weight Can Affect Effectiveness
Some research has found that Plan B is less effective in people who weigh more than 165 pounds, with some studies showing up to double the risk of pregnancy compared to someone at a lower weight. Other studies have been less conclusive, and the research overall is mixed.
The FDA has not issued specific weight-based recommendations, and experts are clear that people over 165 pounds should still take Plan B if they need it. It remains better than not taking anything. However, if you weigh more than 165 pounds and you’re within five days of unprotected sex, a prescription alternative called ella (ulipristal acetate) may be a stronger option to discuss with a pharmacist or provider.
When Ella Is the Better Choice
Ella is a different type of emergency contraception that works effectively for the full five-day window. It’s more effective than Plan B at every time point, but particularly between 72 and 120 hours after unprotected sex. If you’re past the three-day mark, ella is the better pharmaceutical option.
The tradeoff is access: ella requires a prescription in most states, while Plan B is available over the counter with no age restriction. If speed matters and Plan B is what’s available, take it. If you have a day to get a prescription and you’re within the five-day window, ella gives you better odds.
Keeping Plan B on Hand
Because timing is the single biggest factor in effectiveness, many people choose to keep a box of Plan B at home. If you do, check the expiration date every year or so and replace it before it lapses. Store it at room temperature, away from moisture and heat. The pill is stable for years under normal conditions, so there’s no special handling required. Just verify the date is still good when the moment arrives.