Take Plan B as soon as possible after unprotected sex. It is most effective within the first 24 hours, but it can work up to 72 hours (3 days) later, with decreasing effectiveness as time passes. There is some limited protection between 72 and 120 hours (5 days), though other emergency contraception options work better in that later window.
Why Sooner Is Better
Plan B works by delaying or preventing ovulation, the release of an egg from the ovary. If your body hasn’t ovulated yet, the pill can pause that process so there’s no egg available to be fertilized. But once ovulation has already happened, Plan B has no meaningful effect on pregnancy rates. That’s why speed matters: the sooner you take it, the more likely you are to catch your body before ovulation occurs.
Overall, Plan B prevents 81 to 90 percent of expected pregnancies, with that range depending almost entirely on how quickly you take it. Within the first 24 hours, effectiveness is at the high end. By 48 to 72 hours, it drops noticeably. And between 72 and 120 hours, it still offers some protection, but significantly less.
What Happens After 72 Hours
Plan B’s label focuses on the 72-hour window, but CDC guidelines note that emergency contraceptive pills can be taken up to 5 days (120 hours) after unprotected sex. If you’re past the 3-day mark, though, a different emergency contraceptive pill containing ulipristal acetate (sold as ella) is more effective in that 72-to-120-hour range. A copper IUD, placed by a provider within 5 days, is the most effective emergency contraception option at any point in that window and can then serve as ongoing birth control.
Body Weight Affects How Well It Works
Plan B is less effective for people at higher body weights. Research from Oregon Health & Science University found that individuals with a BMI of 30 or above experienced contraceptive failure four times as often as those with a BMI under 25. The reason is straightforward: blood levels of the active drug were about 50 percent lower in people with a BMI of 30 after a standard dose, meaning the drug never reaches the concentration needed to reliably block ovulation. Doubling the dose did not solve the problem.
If your BMI is 30 or higher, ulipristal acetate or a copper IUD are more reliable options. The CDC notes that Plan B may be less effective than ulipristal acetate specifically for people with obesity.
Medications That Reduce Effectiveness
Certain medications speed up how quickly your liver breaks down Plan B, lowering the amount of active drug in your bloodstream. These include some treatments for epilepsy (such as carbamazepine and phenytoin), tuberculosis medications (such as rifampicin), some HIV medications, and the antifungal griseofulvin. The herbal supplement St. John’s wort has the same effect. One HIV medication, efavirenz, cuts Plan B blood levels by roughly 50 percent.
This isn’t limited to medications you’re currently taking. Elevated enzyme levels can persist for up to four weeks after stopping one of these drugs. If any of these apply to you, a copper IUD or ulipristal acetate may be a better choice.
What to Expect After Taking It
Plan B is a single pill taken by mouth. If you vomit within three hours of swallowing it, you may need to take another dose, so contact a pharmacist or healthcare provider in that situation.
Your next period may come a few days early or a few days late. This is normal and happens because the pill temporarily disrupts your hormonal cycle. If your period is more than a week late, take a pregnancy test. Spotting between periods is also common in the days following the dose.
Other typical side effects include nausea, fatigue, headache, and lower abdominal cramping. These generally resolve within a day or two.
Safety During Breastfeeding
Plan B is considered safe while breastfeeding. The active drug does pass into breast milk, but in minimal quantities. Studies comparing breastfeeding outcomes between people who took Plan B and those who didn’t found no differences. There is no recommendation to pump and discard milk after taking it.
Repeated Use
Taking Plan B more than once in a cycle or multiple times over several months is not harmful, but it’s less effective than regular contraception. Each use works independently, so a previous dose does not protect you from a later instance of unprotected sex. If you find yourself needing emergency contraception frequently, a daily pill, IUD, implant, or other ongoing method will provide far more reliable protection.