Plan B can be taken up to 72 hours (three days) after unprotected sex, but it works best the sooner you take it. At 24 hours, it prevents about 94% of expected pregnancies. By 72 hours, that number drops to roughly 58%. Every hour you wait reduces your chances of preventing pregnancy.
How Effectiveness Drops Over Time
The 72-hour window is the official cutoff, but thinking of it as a hard deadline can be misleading. Plan B’s effectiveness doesn’t hold steady for three days and then suddenly stop working. It declines continuously from the moment you have unprotected sex. The sharpest drop happens between 48 and 72 hours.
Here’s what the numbers look like:
- Within 24 hours: about 94% effective
- Within 72 hours: about 58% effective
That’s a significant gap. If you’re debating whether to take it tonight or tomorrow morning, tonight is the better choice.
Why Timing Matters So Much
Plan B works by delaying or preventing ovulation. It contains a synthetic hormone that suppresses the surge your body uses to trigger egg release. If you take it before that surge happens, it can effectively hit pause on ovulation long enough for sperm to die off (sperm survive about five days in the reproductive tract).
Here’s the critical part: if ovulation has already occurred, Plan B does not work. FDA-reviewed data shows that when the pill is taken after ovulation, the pregnancy rate is the same as if nothing had been taken at all. This is why timing is everything. The longer you wait, the greater the chance your body has already released an egg, and at that point the pill can’t help.
If You’re Past 72 Hours
If more than three days have passed, Plan B is no longer your best option, but you still have two alternatives that work up to five days (120 hours) after unprotected sex.
The first is a prescription pill called ella (ulipristal acetate). Unlike Plan B, ella maintains consistent effectiveness across the full five-day window. In the 72 to 120 hour range specifically, ella significantly outperforms Plan B. In one study of over 200 women who received emergency contraception in that late window, three pregnancies occurred with Plan B and none with ella. You’ll need a prescription or a telehealth visit to get it.
The second option is a copper IUD, which can be placed by a healthcare provider within five days of unprotected sex. In some cases, if the day of ovulation can be estimated, placement can happen even beyond five days after sex, as long as it’s within five days of ovulation. The copper IUD is the most effective form of emergency contraception available, and it doubles as long-term birth control for up to 10 years.
Body Weight Can Affect How Well It Works
Plan B becomes less effective if you weigh more than 165 pounds. The active ingredient is dosed as a single fixed tablet, and higher body weight reduces the drug’s concentration in your system. If you’re above that weight threshold, ella or a copper IUD are more reliable options worth discussing with a pharmacist or provider.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down Plan B’s active ingredient, leaving less of it available to do its job. These include some medications used for epilepsy, tuberculosis, HIV, and fungal infections. St. John’s wort, a common herbal supplement, has the same effect. One HIV medication (efavirenz) cuts Plan B’s blood levels by about 50%.
This interaction doesn’t stop the moment you quit the medication, either. The enzyme changes in your liver can persist for up to four weeks after you stop taking the interfering drug. If any of these apply to you, a copper IUD is the most reliable emergency option.
Taking Plan B More Than Once
There’s no medical limit on how many times you can take Plan B in a single cycle. Frequent use isn’t associated with long-term side effects or complications. However, taking a second dose after an initial dose on the same day won’t boost effectiveness. You only need a new dose if you have another episode of unprotected sex on a separate occasion.
One important exception: don’t take Plan B if you’ve already taken ella since your last period. The two drugs can counteract each other and leave you with less protection than either pill would provide alone. Keep at least five days between them.
Common Side Effects
The most common side effects are nausea, headache, and period-like cramping. These are generally mild and short-lived. If you vomit within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose. Check with a pharmacist if that happens.
Your next period may arrive a few days early or late. Some spotting between periods is also normal. If your period is more than a week late, take a pregnancy test.
Getting Back on Regular Birth Control
If you took Plan B, you can start or restart your regular hormonal birth control (the pill, patch, or ring) immediately. You don’t need to wait. But you’ll need to use condoms or abstain for the first seven days, since your regular method won’t be fully effective right away.
The rule is different if you took ella instead. In that case, wait until the sixth day after taking it before starting hormonal birth control, because the hormones in your regular method can interfere with ella’s ability to work. Use a backup method during that gap.