Plan B is most effective within the first 24 hours after unprotected sex, and its effectiveness drops steadily from there. It can reduce the risk of pregnancy when taken up to 72 hours (3 days) afterward, with some residual effect through 4 to 5 days, though pregnancy rates rise noticeably after the 3-day mark.
Effectiveness by Time Window
The sooner you take Plan B, the better it works. Within the first 24 hours, it’s at its highest effectiveness. After 48 hours, the effect starts to decline more sharply. A meta-analysis of levonorgestrel (the active ingredient in Plan B) found that pregnancy rates remained low when the pill was taken within 4 days but increased at the 4-to-5-day mark.
Both Plan B and its prescription alternative, Ella (ulipristal acetate), perform similarly when taken within the first 3 days. The critical difference shows up between days 3 and 5: Ella maintains consistent effectiveness through the full 120-hour window, while Plan B’s reliability fades. In one study of over 200 women who took emergency contraception in that 72-to-120-hour range, three pregnancies occurred in the Plan B group and none in the Ella group.
If you’re past the 72-hour mark, Ella is the stronger option, but it requires a prescription.
How Plan B Actually Works
Plan B works by stopping or delaying the release of an egg from the ovary. That’s its only mechanism. The FDA has concluded that it does not affect fertilization or implantation, and it cannot end an existing pregnancy. It simply prevents ovulation from happening during the window when sperm could still be viable.
This is also why timing matters so much. If ovulation has already occurred before you take the pill, Plan B has little to no effect. It’s working against a biological clock: sperm can survive in the reproductive tract for up to 5 days, so the pill needs to delay the egg’s release long enough that no viable sperm remain when ovulation eventually happens.
Body Weight Can Reduce Effectiveness
Plan B becomes significantly less reliable at higher body weights. Research from Oregon Health & Science University found that people with a BMI of 30 or above experienced morning-after pill failure four times as often as those with a BMI under 25. The reason is straightforward: blood levels of levonorgestrel were about 50% lower in individuals with a BMI of 30 after taking a standard dose, meaning the drug never reaches the concentration needed to reliably prevent ovulation.
Doubling the dose doesn’t solve the problem. The same research team tested a double dose in people with BMIs above 30 and found it was not effective. If your BMI is 30 or higher, Ella or a copper IUD (which can be inserted up to 5 days after unprotected sex) are more reliable options worth discussing with a provider.
Taking Plan B More Than Once
There’s no medical limit on how many times you can take Plan B. Repeated use of levonorgestrel-based emergency contraception isn’t associated with long-term side effects or health complications. If you have unprotected sex on two separate occasions in the same cycle, you can take Plan B each time.
A few important caveats, though. Taking an extra dose after the first one won’t make a single instance more effective. One dose is the full dose. Also, you should not take Plan B and Ella within 5 days of each other, because the two drugs can counteract each other and leave you unprotected. If you’ve already taken Ella during your current cycle, don’t follow it with Plan B.
What to Expect Afterward
Most people experience mild or no side effects. Nausea is the most commonly reported one. Some people notice spotting, a heavier or lighter period, or a period that arrives a few days early or late. These changes are temporary and result from the hormone surge that delays ovulation.
Your next period is your first real signal that Plan B worked. If it’s more than a week late, take a pregnancy test. Even if your period timing seems normal, Planned Parenthood recommends taking a pregnancy test 3 weeks after you took the pill to confirm you’re not pregnant. Standard home tests are accurate at that point.
Starting Regular Contraception After Plan B
You can start or resume a regular birth control method immediately after taking Plan B. The pill, patch, ring, shot, or hormonal IUD can all be initiated the same day without reducing the effectiveness of either the emergency contraception or the new method. You’ll need to use condoms or abstain for the next 7 days while your regular method takes effect.
This is another area where Plan B and Ella differ. If you took Ella, you need to wait before starting hormonal contraception, because hormonal methods can interfere with how Ella works. Plan B doesn’t have that limitation since it’s a progestin-only formulation without anti-progestin properties.