Plan B works best when taken as soon as possible after unprotected sex, and the absolute deadline is 72 hours (three days). Every hour you wait reduces its effectiveness, so taking it within the first 24 hours gives you the strongest protection. If more than 72 hours have passed, Plan B is no longer a reliable option, but a different emergency contraceptive called ella can work up to 120 hours (five days) after sex.
Why Sooner Is Better
Plan B contains a synthetic hormone that works by delaying or preventing your ovary from releasing an egg. If your body hasn’t ovulated yet, the pill essentially puts that process on pause so there’s no egg available to be fertilized. Once ovulation has already happened, Plan B becomes much less effective. It does not end a pregnancy that has already started.
Because ovulation can happen at any point in your cycle, time is the biggest factor in whether the pill works. Taking it within the first 12 to 24 hours gives you the highest chance of preventing pregnancy. By 48 to 72 hours, the odds are noticeably lower. The simplest rule: take it as soon as you can get your hands on it.
What If It’s Been More Than 72 Hours
If you’re past the three-day window, ella is a stronger alternative. It can be taken up to five days (120 hours) after unprotected sex, and it’s more effective than Plan B at every time point within that range. The tradeoff is that ella requires a prescription in the U.S., while Plan B is available over the counter at pharmacies without an age restriction.
A copper IUD is another option that works as emergency contraception for up to five days after sex, and it’s the single most effective form of emergency contraception available. It requires a clinic visit for insertion, but it then doubles as long-term birth control for years.
Weight Can Affect How Well It Works
Plan B becomes less effective if you weigh more than 165 pounds. This doesn’t mean it won’t work at all, but the protection drops enough that alternatives are worth considering. Ella remains effective at higher weights, though its efficacy also decreases above 195 pounds. If weight is a concern, a copper IUD is unaffected by body size and offers the most reliable emergency protection.
Common Side Effects
Most people experience mild side effects that resolve on their own. The most common ones include nausea, headache, fatigue, dizziness, breast tenderness, and cramping in the lower abdomen. Some people also get light spotting. These effects are generally short-lived and don’t require treatment.
The bigger change you’ll likely notice is a shift in your next period. Emergency contraception commonly delays or advances your cycle, so your period may come earlier or later than expected. It may also be lighter, heavier, or just feel a bit different than usual. This is normal and not a sign that something went wrong.
When to Take a Pregnancy Test
If your period doesn’t arrive within a week or so of when you’d normally expect it, take a pregnancy test. If your cycle is irregular and you’re not sure when your period is due, wait at least 21 days after the unprotected sex before testing. Testing too early can give a false negative because the pregnancy hormone needs time to build to detectable levels.
Taking Plan B More Than Once
Taking Plan B multiple times, even within the same menstrual cycle, is safe. It won’t cause long-term side effects or reduce its effectiveness the next time you use it. That said, it’s not designed as a routine method of birth control. It’s less effective than daily pills, IUDs, or implants, and repeated use can make your cycle unpredictable. If you’re reaching for emergency contraception frequently, a regular contraceptive method will give you better, more consistent protection.
Keeping Plan B on Hand
Plan B has a shelf life of four years, which makes it practical to buy in advance and store at home. Keep it in a cool, dry place at room temperature (between 68 and 77 degrees Fahrenheit). A bedroom drawer or closet works well. Avoid the bathroom, where humidity and heat from showers can degrade the medication over time. Having a pill already in your drawer eliminates the trip to the pharmacy and lets you take it sooner, which is the single biggest factor in whether it works.