Plan B works best when taken as soon as possible after unprotected sex, ideally within the first 24 hours, when it’s about 94% effective. It can be taken up to 72 hours (three days) afterward, but by that point effectiveness drops to roughly 58%. Every hour matters, so treating it as a “morning-after” pill you can take at your leisure understates the urgency.
How Effectiveness Changes Over Time
The clock starts ticking immediately after unprotected sex. At 24 hours, Plan B prevents pregnancy about 94% of the time. By 48 hours, that number has already fallen noticeably, and at the 72-hour mark it’s down to about 58%. The pill is labeled for use up to 72 hours, but “up to” is doing a lot of work in that sentence. If you’re deciding between taking it tonight and taking it tomorrow morning, take it tonight.
There is no benefit to waiting. The drug doesn’t need to be timed to a meal or a certain hour of the day. If you have it on hand, take it immediately.
Why Timing Matters So Much
Plan B contains a synthetic hormone called levonorgestrel that works by delaying or preventing ovulation, the release of an egg from the ovary. It’s effective only if your body hasn’t ovulated yet. Once the egg has been released, the pill does not reduce the chance of pregnancy.
This is why speed matters. The sooner you take it, the more likely you catch your body before ovulation happens. If you’re early in your cycle (before the midpoint), you have a better chance of the pill working. If you’re right around the time you’d normally ovulate, the window may already be closing regardless of how quickly you act. You can’t know the exact timing of ovulation without monitoring, which is why taking the pill fast gives you the best odds no matter where you are in your cycle.
What If It’s Been More Than 72 Hours
If three days have passed, Plan B is not your best option. A different emergency contraceptive pill, sold under the brand name ella, works for up to 120 hours (five days) after unprotected sex. In clinical trials comparing the two drugs in women who took emergency contraception between 72 and 120 hours, all the pregnancies occurred in the levonorgestrel (Plan B) group, with none in the ella group. Ella requires a prescription, so you’d need to contact a provider quickly.
A copper IUD is another option that works up to five days after unprotected sex and is the most effective form of emergency contraception available, preventing pregnancy more than 99% of the time. It also doubles as long-term birth control once placed.
Body Weight Can Reduce Effectiveness
If you weigh more than 165 pounds, Plan B may not work as well. The hormone concentration in the standard dose appears to be less effective at higher body weights. This doesn’t mean it’s useless, but the reduced effectiveness is significant enough that Planned Parenthood flags it as a consideration. If this applies to you, ella or a copper IUD may be more reliable choices, and it’s worth talking to a pharmacist or provider about which option makes sense given your timeline.
Common Side Effects
Most side effects from Plan B are mild and resolve within 24 hours. Nausea is the most frequently reported, along with fatigue, headache, lower abdominal pain, and dizziness. Some people experience breast tenderness or diarrhea. If you vomit within two hours of taking the pill, you may need another dose since the medication might not have been fully absorbed.
Your next period may arrive up to a week later than expected. Some people notice spotting in the days or weeks after taking the pill. If your period doesn’t come within three weeks, take a pregnancy test.
Can You Take It More Than Once in a Cycle
Yes. Plan B can be used more than once in the same menstrual cycle if needed. The American College of Obstetricians and Gynecologists endorses repeated use, and studies have not found an increased risk of serious side effects from multiple doses in the same cycle. You may experience more menstrual irregularity, like unexpected spotting or a shifted period, but these changes are typically mild. That said, if you’re finding yourself reaching for emergency contraception frequently, a regular method of birth control will be more effective and less disruptive to your cycle.