Plan B is between 58% and 94% effective at preventing pregnancy, depending on how quickly you take it after unprotected sex. That range is wide because timing matters enormously. Taken within the first 24 hours, studies show it prevents about 94% of expected pregnancies. By the 72-hour mark, that number drops to roughly 58%.
How Timing Changes the Numbers
The sooner you take Plan B, the better it works. Here’s how the effectiveness breaks down:
- Within 24 hours: approximately 94% effective
- Within 48 hours: around 85% effective
- Within 72 hours: approximately 58% effective
- 73 to 120 hours: still possible to take, but significantly less effective
These percentages describe how many expected pregnancies are prevented, not your individual chance of getting pregnant. Researchers calculate this by comparing how many pregnancies occurred among women who took the pill to how many pregnancies would have been expected without any intervention, based on where those women were in their menstrual cycles. If 100 pregnancies would have been expected and only 6 occurred, that’s 94% effectiveness.
This is an important distinction. It doesn’t mean you have a 6% chance of getting pregnant after taking Plan B within 24 hours. Your actual risk depends on whether you were near ovulation in the first place. Most unprotected sex doesn’t result in pregnancy even without emergency contraception, because conception is only possible during a narrow fertile window each cycle.
Why Plan B Stops Working Near Ovulation
Plan B contains a synthetic hormone called levonorgestrel. It works by interfering with the hormonal signal that triggers your ovary to release an egg. Specifically, it suppresses or delays the surge of a hormone that kicks off ovulation. If that surge hasn’t started yet, Plan B can pause the process long enough for sperm (which survive about five days) to die off before an egg ever appears.
But once that hormonal surge is already underway, Plan B can no longer stop it. If ovulation has already happened or is actively happening, the pill has no way to prevent the egg from being fertilized. This is the single biggest reason Plan B sometimes fails: it’s not that the pill didn’t work properly, it’s that ovulation had already passed the point of no return.
This also explains why speed matters so much. The longer you wait, the more likely it is that ovulation will begin on its own before the pill can intervene.
Body Weight Affects Effectiveness
Plan B becomes less effective at higher body weights. According to Planned Parenthood, if you weigh more than 165 pounds, levonorgestrel-based emergency contraception won’t work as well. The pill isn’t necessarily useless above that weight, but the hormone may not reach high enough levels in your bloodstream to reliably delay ovulation.
If this applies to you, there are two stronger alternatives. One is a prescription emergency contraceptive pill called ella (ulipristal acetate), which maintains higher effectiveness across a wider weight range and can be up to 98% effective within 24 hours. The other is a copper IUD, which a provider can insert up to five days after unprotected sex. The copper IUD is the most effective form of emergency contraception available, with pregnancy rates at or near zero in clinical trials, and it doubles as long-term birth control for up to 10 years.
Medications That Reduce Plan B’s Effectiveness
Certain medications speed up how fast your liver breaks down levonorgestrel, which can cut its blood levels dramatically. One HIV medication, efavirenz, reduces levonorgestrel levels by about 50%. Several categories of drugs cause this same problem:
- Some epilepsy medications (including phenytoin and carbamazepine)
- Some tuberculosis medications (including rifampicin)
- Some HIV medications
- St. John’s wort, a common herbal supplement
The effect isn’t limited to active use. Elevated enzyme levels can persist for up to four weeks after you stop taking these medications, meaning Plan B could still be less effective even if you recently discontinued one of them. If you’re on any of these, a copper IUD is a more reliable emergency option.
How Plan B Compares to Other Options
Plan B is the most accessible emergency contraceptive because it’s available over the counter without a prescription or age restriction. But it’s not the most effective one. Ella, which requires a prescription, works better across the full five-day window and at higher body weights. Clinical data shows ella can reach 98% effectiveness when taken within 24 hours, and it maintains stronger protection through day five compared to Plan B’s steep drop-off after 72 hours.
The copper IUD outperforms both pills. In head-to-head trials comparing levonorgestrel to the copper IUD for emergency contraception, the copper IUD group had a 0% pregnancy rate while the levonorgestrel group had a 0.3% pregnancy rate at one month. The copper IUD works by creating an environment in the uterus that is inhospitable to both sperm and fertilized eggs, so it remains effective regardless of where you are in your cycle, including after ovulation has already occurred.
What to Expect After Taking It
Plan B commonly causes nausea, fatigue, headache, and breast tenderness. These side effects are temporary and typically resolve within a day or two. Your next period may come earlier or later than expected, sometimes by a week or more. Heavier or lighter bleeding than usual is also common. If your period is more than a week late, take a pregnancy test.
Taking Plan B does not affect your fertility going forward. It also doesn’t protect you from pregnancy for the rest of your cycle. If you have unprotected sex again after taking it, even just a day or two later, you’re at risk of pregnancy from that new exposure.