Plan B works best within the first 24 hours after unprotected sex, when it’s about 94% effective. The maximum recommended window is 72 hours (three days), but by that point effectiveness drops to roughly 58%. Every hour counts, so taking it as soon as possible makes a real difference.
How Effectiveness Changes Hour by Hour
Plan B contains a single dose of a synthetic hormone that works primarily by delaying or blocking ovulation. If your body hasn’t released an egg yet, the pill prevents that release, and pregnancy can’t happen. But if ovulation has already occurred, Plan B has little to no effect. Research shows that women who took it after the hormonal surge that triggers ovulation had conception rates similar to those who took a placebo.
That’s why timing matters so much. At 24 hours, you’re catching most cases before ovulation can happen. By 72 hours, the window is closing. The numbers tell a clear story:
- Within 24 hours: approximately 94% effective
- Within 72 hours: approximately 58% effective
After 72 hours, Plan B is not recommended because its reliability drops significantly. There’s no good data supporting its use between 72 and 120 hours.
If You’re Past the 72-Hour Window
You still have options. A prescription pill called ella (ulipristal acetate) works for up to five days (120 hours) after unprotected sex and is more effective than Plan B at every time point, including the first 24 hours. It requires a prescription, but many telehealth services can issue one quickly.
The most effective emergency contraception available is a copper IUD, inserted by a healthcare provider within five days. In clinical trials, copper IUDs prevented 100% of expected pregnancies among participants, with zero failures across 318 users. It also doubles as long-term birth control for up to 10 years afterward, so it serves two purposes at once.
Body Weight Affects How Well Plan B Works
This is something many people don’t realize. 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. Blood levels of the active hormone were about 50% lower in the higher-BMI group, meaning the drug never reached the concentration needed to block ovulation. Doubling the dose didn’t solve the problem either.
If your weight is above 176 pounds or your BMI is 30 or higher, ella or a copper IUD are more reliable choices. This isn’t about body shaming. It’s straightforward pharmacology: the drug dilutes in a larger body and doesn’t reach an effective threshold.
Can You Take Plan B More Than Once?
Yes. There’s no medical limit on how many times you can use Plan B. Taking it multiple times doesn’t reduce its effectiveness for future use, and it won’t cause long-term side effects. A 2022 review of 33 studies found no impact on fallopian tube function, ectopic pregnancy risk, miscarriage rates, or future fertility.
That said, using it frequently comes with practical downsides. The most common issue is menstrual irregularity. Your period may come early, late, or be heavier or lighter than usual, and if you’re taking Plan B repeatedly, those disruptions can stack up and make your cycle unpredictable for a while. It’s also significantly more expensive than regular contraception over time. Plan B is a backup, not a primary strategy, simply because other methods prevent pregnancy far more reliably when used consistently.
What to Expect After Taking It
Common side effects include nausea, fatigue, headache, and breast tenderness. These typically resolve within a day or two. Your next period may arrive up to a week earlier or later than expected. If your period is more than a week late, a pregnancy test is a good idea.
Plan B does not end an existing pregnancy. It only works by preventing ovulation before fertilization occurs. If implantation has already happened, the pill has no effect. Nine out of ten studies examining this question found no difference in the uterine lining’s ability to support implantation after exposure to the drug compared to controls.