Plan B is significantly less effective at 4 days (96 hours) compared to taking it within the first 24 hours, and its labeled window is only 72 hours. That said, it doesn’t drop to zero. CDC-reviewed studies show that levonorgestrel (the active ingredient in Plan B) still reduces pregnancy risk through day 5, though rates of pregnancy are noticeably higher when taken after 3 days. If you’re at the 4-day mark, a different emergency contraceptive is likely a better choice.
Why Effectiveness Drops After 72 Hours
Plan B works by preventing or delaying ovulation. It does not end an existing pregnancy. This means its success depends almost entirely on timing relative to your ovulation, not just timing relative to sex. If you haven’t ovulated yet when you take it, the pill can delay the release of an egg long enough to prevent fertilization. If ovulation has already happened, Plan B has little to no effect.
The longer you wait, the more likely it is that ovulation has already occurred or is imminent, which is why effectiveness declines sharply with each passing day. By day 4, the window where Plan B can still intervene is narrow.
How Plan B Compares to Ella at 4 Days
A large randomized trial published in The Lancet compared Plan B directly to ella (a prescription emergency contraceptive) in women who took the pill between 72 and 120 hours after sex. Among 203 women in that late window, all three pregnancies that occurred were in the Plan B group. None were in the ella group. That’s a small sample, but the pattern is consistent with what pharmacologists would expect: ella works closer to the moment of ovulation and maintains its effectiveness further into the 5-day window than Plan B does.
Ella is available by prescription in the U.S. and requires a visit to a provider or a telehealth consultation. If you’re reading this at the 4-day mark and haven’t taken anything yet, ella is the stronger pill-based option.
The Copper IUD Is the Most Effective Late Option
The most reliable form of emergency contraception at 4 days is a copper IUD inserted by a healthcare provider. It can be placed up to 5 days after unprotected sex and has a failure rate of less than 1%, regardless of where you are in your cycle. Unlike pills that work by delaying ovulation, the copper IUD creates an environment in the uterus that prevents fertilization and implantation, so it remains effective even after ovulation has occurred.
The copper IUD also doubles as long-term birth control for up to 10 years if you choose to keep it. The insertion process takes a few minutes and can cause cramping, but it’s the closest thing to a guarantee available in the emergency contraception category.
Body Weight Affects Plan B’s Reliability
Even within the recommended 72-hour window, Plan B becomes less reliable as body weight increases. Research shows a steep rise in pregnancy risk starting around 70 to 75 kilograms (roughly 154 to 165 pounds), reaching a failure rate of 6% or higher at around 80 kilograms. For women weighing more than 70 kg or with a BMI above 26, clinical guidelines note higher failure rates.
If you weigh more than about 155 pounds and you’re already past the 3-day mark, Plan B is a particularly poor bet. Ella performs somewhat better at higher weights, though its effectiveness also starts to decline above 85 kg. The copper IUD is unaffected by weight.
What to Expect After Taking It Late
Taking Plan B at any point in your cycle is safe, even past the labeled 72-hour window. Common side effects include nausea, fatigue, headache, and light spotting. Your next period may arrive up to a week later than expected, which can cause anxiety about whether you’re pregnant. This delay is a normal hormonal effect of the medication, not a sign of pregnancy on its own.
If your period is more than a week late, take a pregnancy test. If you’re unsure when your period is due, the NHS recommends testing at least 21 days after the unprotected sex in question. Home pregnancy tests are highly accurate by that point.
A Realistic Look at Your Options Right Now
If you’re at or near the 96-hour mark, here’s how your options stack up in terms of effectiveness:
- Copper IUD: Less than 1% failure rate through day 5. Requires a provider visit.
- Ella: Maintains strong effectiveness through day 5. Requires a prescription.
- Plan B: Still reduces risk somewhat through day 5, but with noticeably higher failure rates compared to the first 3 days. Available over the counter.
Taking Plan B at 4 days is better than taking nothing, but it’s the weakest option available at this point. If you can access ella or a copper IUD within the next 24 hours, either one gives you meaningfully better protection.