Plan B is officially approved for use within 72 hours (3 days) of unprotected sex, but it starts losing effectiveness well before that cutoff. The sooner you take it, the better it works. Taking it within the first 24 hours gives you the strongest protection, and every day you wait reduces your odds.
How Effectiveness Drops Hour by Hour
When taken within 72 hours, Plan B prevents pregnancy about 87% to 90% of the time. That number represents an average across the full three-day window, though, and it masks a steep decline. Taking it in the first 12 to 24 hours is significantly more effective than waiting until hour 60 or 70.
After 72 hours, the picture gets worse. Studies show that between 72 and 120 hours (3 to 5 days), effectiveness drops to roughly 72% to 87%, and the pregnancy rate nearly doubles, rising from 0.8% to 1.8%. Some evidence suggests Plan B can still offer moderate protection up to 5 days after unprotected sex, which is why some guidelines mention a 120-hour window. But “moderate protection” is a long way from reliable, and 5 days is the absolute outer limit where any benefit has been observed.
After 5 days, Plan B has no demonstrated ability to lower your chances of pregnancy.
Why Timing Matters More Than the Clock
Plan B works primarily by delaying or preventing ovulation. It stops your body from releasing an egg, so sperm that’s already present has nothing to fertilize. This is why it’s so time-sensitive: if ovulation has already happened, Plan B loses its main mechanism of action.
Research bears this out dramatically. When taken during the pre-ovulatory window, levonorgestrel (the active ingredient in Plan B) is highly effective at blocking ovulation. But when taken on the day of ovulation or after, its ability to suppress ovulation drops to essentially zero. This means two people could take Plan B at the exact same number of hours after sex and get very different results depending on where each person is in their cycle. If you’re days away from ovulating, Plan B has time to intervene. If you’re right at the point of ovulation, it likely won’t help regardless of how quickly you take it.
Body Weight Affects How Well It Works
Plan B may be less effective for people who weigh more than 165 pounds, and potentially ineffective for those over 176 pounds. Health Canada is the only major regulator that currently includes this guidance on the label, but the concern is supported by pharmacological data: levonorgestrel is a fixed-dose pill, and higher body weight can dilute its concentration in the bloodstream.
A BMI over 25 appears to be the threshold where effectiveness starts to decline. For people with a BMI over 30, the drop-off may be more significant. This doesn’t mean Plan B is worthless at higher weights, and medical organizations recommend not being discouraged from taking it if no other option is available. But if you have access to alternatives, they’re worth considering.
Better Options If You’re Past 72 Hours
If it’s been more than three days since unprotected sex, you have two options that work better than Plan B in that window.
Ella (ulipristal acetate) is a prescription emergency contraceptive pill that remains effective for a full 5 days. Unlike Plan B, its effectiveness holds relatively steady across that window. Pregnancy rates in studies were 2.3% when taken between 48 and 72 hours, 2.1% between 72 and 96 hours, and 1.3% between 96 and 120 hours. That consistency is a major advantage over Plan B, which drops off sharply after day two. The CDC notes that Ella is more effective than Plan B in the 3-to-5 day range, and it also performs better for people with obesity. You’ll need a prescription, which you can sometimes get through telehealth services.
The copper IUD is the most effective form of emergency contraception available. It can be inserted up to 5 days after unprotected sex, and when the timing of ovulation can be estimated, it may be placed even later as long as insertion happens within 5 days of ovulation. Its failure rate as emergency contraception is less than 1%, making it far more reliable than any pill. It also doubles as long-term birth control, lasting up to 10 years. The tradeoff is that it requires a clinical visit for insertion.
What to Expect After Taking Plan B
Common side effects include nausea, fatigue, headache, and breast tenderness. These typically resolve within a day or two. The most noticeable effect for many people is a change in their next period. Plan B can delay your period by up to a week, and it may also cause spotting or a heavier or lighter flow than usual.
If your period is more than three weeks late after 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 warrants checking. Home pregnancy tests are accurate by that point and will give you a reliable answer.