Plan B is still effective after 2 days, but it’s already losing potency. When taken within the first 24 hours, it prevents roughly 85 to 95% of expected pregnancies. By the 48-hour mark, that number is still relatively high, with one large WHO study finding it prevented about 94% of expected pregnancies when taken between 25 and 48 hours. But by day three (49 to 72 hours), effectiveness drops sharply to somewhere between 58% and 77%, depending on the study. The bottom line: two days in, Plan B still works well, but every additional hour matters.
How Effectiveness Drops Hour by Hour
Plan B’s active ingredient is a synthetic hormone that works primarily by delaying or blocking ovulation. That delay only helps if ovulation hasn’t happened yet, which is why timing is everything. The clock starts at the moment of unprotected sex, not the next morning.
Here’s how the numbers break down from a large clinical trial reviewed by the FDA:
- 0 to 24 hours: Prevented about 80% of expected pregnancies
- 25 to 48 hours: Prevented about 94% of expected pregnancies
- 49 to 72 hours: Prevented about 77% of expected pregnancies
The Cleveland Clinic cites a slightly different range, noting about 94% effectiveness within the first 24 hours and roughly 58% by 72 hours. The variation between studies comes down to differences in study populations and how effectiveness was measured, but the overall pattern is consistent: sooner is always better, and the steepest drop-off happens after the second day.
Why Timing Matters So Much
Plan B’s main job is to prevent or delay the release of an egg from the ovary. If it can push ovulation back by even a few days, sperm from the unprotected encounter die off before they ever meet an egg. Sperm typically survive three to five days inside the reproductive tract, so the race is between the drug delaying ovulation and the sperm reaching a viable egg.
This is also why Plan B has a hard limit. Research shows that if ovulation has already occurred, Plan B has essentially no effect on preventing pregnancy. One study found that among women who took it on the day of ovulation or afterward, the pregnancy rate was almost identical to what would have been expected without any intervention at all. At 48 hours, your odds are better simply because ovulation is less likely to have happened yet, but there’s no way to know for certain where you are in your cycle without medical testing.
Body Weight Can Reduce Effectiveness
Plan B may work less well for people at higher body weights. Health Canada has stated that levonorgestrel may be less effective in women over 165 pounds and potentially ineffective over 176 pounds. Other health organizations have pushed back on calling it completely ineffective at those weights, but the trend in the data is clear: effectiveness decreases as weight increases.
For people with a BMI over 30, the risk of unintended pregnancy after taking Plan B is roughly 6%, compared to about 2.5% with a prescription alternative called ella (ulipristal acetate). If you’re in this weight range and already 48 hours past unprotected sex, talking to a pharmacist or provider about ella may be worth considering, since it maintains stronger effectiveness further from the event.
How Ella Compares at the 48-Hour Mark
Ella is a prescription-only emergency contraceptive that works for up to 5 days (120 hours) after unprotected sex. Within the first 3 days, the CDC notes that ella and Plan B have similar effectiveness. The real advantage of ella shows up between days 3 and 5, when Plan B’s effectiveness has dropped significantly but ella’s remains relatively stable.
At the 48-hour mark specifically, both options are reasonable choices. But if you’re closer to 72 hours or beyond, ella is the stronger option. It’s also more effective than Plan B for people with higher BMIs. The tradeoff is that ella requires a prescription, which can add time and cost. Plan B is available over the counter at most pharmacies with no age restriction.
What to Expect After Taking It
Plan B can cause nausea, headache, fatigue, and lower abdominal pain. Some people experience spotting or bleeding within a few days, and your next period may come earlier or later than expected. These side effects are temporary and generally resolve within a day or two.
A late or unusually light period is one of the most common sources of anxiety after taking Plan B. The hormone in the pill can shift your cycle by up to a week in either direction. If your period is more than a week late, a pregnancy test will give you a reliable answer. Planned Parenthood recommends waiting three weeks after taking the pill before testing, since testing too early can produce a false negative.
Copper IUD as a Backup Option
If you’re past the 48-hour mark and want the most effective emergency contraception available, a copper IUD inserted within 5 days of unprotected sex prevents more than 99% of pregnancies. It works regardless of body weight and regardless of where you are in your ovulation cycle, because it changes the uterine environment rather than relying on delaying ovulation. It also doubles as long-term birth control for up to 10 years. The downside is that it requires a provider visit and an insertion procedure, which isn’t always accessible on short notice.