Plan B is most effective within the first 24 hours after unprotected sex, when it prevents pregnancy about 94% of the time. Its effectiveness drops significantly with each passing day, falling to roughly 58% by the 72-hour mark. While the label says to take it within three days, it can still offer some protection up to five days (120 hours) after sex. But “some protection” and “reliable protection” are very different things, and timing isn’t the only factor that matters.
How Effectiveness Drops Over Time
The clock starts ticking immediately after unprotected sex. Here’s how Plan B’s effectiveness changes:
- Within 24 hours: about 94% effective
- Within 72 hours (3 days): about 58% effective
- Between 72 and 120 hours (3 to 5 days): still possible but with lower and less well-documented efficacy
That’s a steep decline. Waiting even one extra day can cut the effectiveness roughly in half. If you’re debating whether to take it now or pick it up tomorrow morning, take it now.
Why Timing Matters So Much
Plan B works by delaying ovulation, the moment your ovary releases an egg. It interrupts the hormonal surge that triggers that release. If you take it before your body has begun that process, it can effectively hit pause and prevent the egg from ever meeting sperm.
Here’s the critical part: if you’ve already ovulated, Plan B does not work. FDA-reviewed clinical data is clear on this point. When taken after ovulation has occurred, the pregnancy rate is the same as if no emergency contraception had been used at all. Plan B has no meaningful effect on what happens after an egg is released.
This is why the pill becomes less effective over time. The longer you wait, the greater the chance that ovulation has already happened or is imminent and can no longer be stopped. If you happen to be early in your cycle and far from ovulation, a later dose may still work. But you typically can’t know exactly where you are in your cycle with enough precision to gamble on it.
Body Weight Affects How Well It Works
Timing isn’t the only variable. Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. The reason is straightforward: blood levels of the active drug were about 50% lower in people with a higher BMI after a standard dose, meaning it likely never reaches the concentration needed to block ovulation.
Doubling the dose doesn’t solve the problem. A study published in Obstetrics & Gynecology found no difference in effectiveness between a single dose and a double dose for participants with a BMI above 30. If your BMI is in that range, a prescription alternative or a copper IUD will be a more reliable option.
Alternatives That Work Later
If you’re past the 72-hour window or concerned about weight-related effectiveness, two alternatives offer stronger protection further out.
A prescription emergency contraceptive pill containing ulipristal acetate (sold as ella) is effective for up to 120 hours, the full five-day window. It’s about 98% effective in the first 24 hours and still around 85% effective at the five-day mark. Unlike Plan B, it can delay ovulation even after the hormonal surge has started, which is why it holds up better at later time points. It does require a prescription.
The copper IUD is the most effective emergency contraceptive available. When inserted within five days of unprotected sex, it prevents pregnancy more than 99% of the time. It requires a clinic visit for placement, but it also doubles as long-term birth control for up to 10 years. For anyone past the three-day mark or with a higher BMI, this is the most reliable choice.
What to Expect After Taking It
Plan B commonly causes nausea, fatigue, headache, and lower abdominal cramping. These side effects are typically mild and resolve within a day or two. Your next period may come earlier or later than expected, and the flow might be heavier or lighter than usual.
If your period hasn’t arrived within three weeks of taking Plan B, take a pregnancy test. A delay of a few days is normal and doesn’t necessarily mean the pill failed, but three weeks without a period is the threshold where testing is recommended.
Plan B does not affect your fertility going forward and is not harmful if you happen to already be pregnant. It simply won’t end an existing pregnancy.