Plan B works best within the first 24 hours after unprotected sex, when it’s roughly 94% effective at preventing pregnancy. Its effectiveness drops steadily from there, falling to about 58% by the 72-hour mark. After 72 hours, it’s generally not considered reliable. A single dose does not provide ongoing protection for any future unprotected sex.
The 72-Hour Window
Plan B is approved for use up to 72 hours (three days) after unprotected sex, but the clock matters more than most people realize. At 24 hours, it reduces the expected pregnancy rate from about 8% down to roughly 1%. By 72 hours, effectiveness has dropped to around 58%. That’s still better than nothing, but it’s a steep decline from where it started.
This is why the common advice to take it “as soon as possible” isn’t just a general suggestion. Every hour you wait translates into a measurable reduction in how well the pill works. If you have access to Plan B and know you need it, taking it immediately rather than the next morning makes a real difference.
How Plan B Prevents Pregnancy
Plan B contains a high dose of a synthetic hormone that works by delaying or stopping ovulation. If your body hasn’t released an egg yet, the pill essentially puts that process on hold long enough for sperm (which survive about five days) to die off before they can fertilize anything.
This is the key to understanding why timing matters so much. If ovulation has already happened, Plan B has very little to offer. Research has confirmed that the pill does not prevent a fertilized egg from implanting in the uterus. In studies where the hormone was given after ovulation had already occurred, pregnancy rates were identical to placebo. It works before fertilization, not after.
This also means Plan B will not affect an existing pregnancy. If implantation has already taken place, the pill has no effect on the embryo.
It Doesn’t Protect You After You Take It
One of the most common misunderstandings about Plan B is that a single dose offers some kind of protective window going forward. It does not. Plan B addresses one specific instance of unprotected sex by interfering with ovulation around the time it’s taken. If you have unprotected sex again the next day, or even later the same day, you’re not covered. You would need another dose for a separate incident.
The hormone in Plan B has a half-life of about 27 hours, meaning half the dose is cleared from your body in just over a day. Within a few days, it’s essentially gone. Whatever ovulation-delaying effect it had is temporary and specific to the moment you took it.
Body Weight Affects How Well It Works
Clinical evidence suggests Plan B becomes less effective at higher body weights. The data isn’t perfectly consistent across studies, but a pooled analysis found that overweight individuals had roughly 1.75 times the odds of the pill failing, and that figure rose to 2.61 times for those with obesity, compared to people at a lower BMI.
The thresholds where effectiveness starts to dip vary by study, but multiple analyses point to reduced reliability starting around 165 pounds (75 kg) or a BMI of 26 or higher. For people with a BMI of 30 or above, a different emergency contraceptive pill (sold under the brand name ella) may maintain better effectiveness, though it requires a prescription. That alternative held a more stable success rate until BMI exceeded 35. The most reliable option regardless of weight is a copper IUD, which can be placed by a provider within five days of unprotected sex and works equally well at any body size.
What to Expect Afterward
Because Plan B delivers a large dose of hormone all at once, it commonly shifts the timing of your next period. Your cycle may come a few days early or a few days late. Some people experience spotting, nausea, fatigue, or breast tenderness in the days following the dose. These side effects are temporary and resolve on their own.
If your period is more than a week late after taking Plan B, a pregnancy test is a reasonable next step. A late period doesn’t necessarily mean the pill failed, but it’s worth confirming.
Restarting Regular Birth Control
If you’re already on a hormonal birth control method and needed Plan B because of a missed pill or other error, you can resume your regular method immediately. There’s no need to wait for your next period. You do, however, need to use a backup method like condoms for the first seven days after restarting, since your regular contraception needs that time to become fully effective again.
If you took ella instead of Plan B, the protocol is different. Because the two drugs can interfere with each other, you should wait six days before starting or restarting any hormonal birth control containing progestin. During that gap, and for seven days after restarting, a backup method is necessary.