Plan B is a single pill containing 1.5 mg of a synthetic hormone that works by delaying or preventing ovulation. When you take it, you’re flooding your body with a large dose of the same type of hormone found in daily birth control pills, which temporarily stops your ovary from releasing an egg. If there’s no egg available, sperm can’t fertilize anything, and pregnancy doesn’t happen.
It does not end an existing pregnancy. If a fertilized egg has already implanted, Plan B will not affect it. A 2022 review of the clinical evidence found no difference in implantation rates after exposure to the drug compared to controls, and conception rates were similar to placebo when the pill was taken after ovulation had already occurred.
How It Works Inside Your Body
Your body naturally releases a surge of hormones right before ovulation to trigger the release of an egg. Plan B disrupts that surge. The synthetic hormone in the pill suppresses the signals your brain sends to your ovaries, effectively pressing pause on the ovulation process. If you haven’t ovulated yet when you take it, the egg stays put and sperm that are waiting in the reproductive tract eventually die off without anything to fertilize.
This is why timing matters so much. If you’ve already ovulated, the pill has little ability to prevent pregnancy because its primary mechanism is no longer available. It won’t block a fertilized egg from implanting.
How Quickly You Need to Take It
Plan B reduces your chance of getting pregnant by 75 to 89 percent when taken within 72 hours (three days) of unprotected sex. The sooner you take it, the better it works, because you’re more likely to catch your body before ovulation happens. After three days, effectiveness drops significantly. You can technically take it up to five days after sex, but it performs much worse in that window.
There is no “sweet spot” hour to aim for. The simple rule is: as soon as possible.
Side Effects You Can Expect
The dose of hormone in Plan B is substantially higher than what’s in a daily birth control pill, so side effects are common. Most people experience at least one of the following in the first day or two: nausea, headache, fatigue, lower abdominal cramping, or breast tenderness. Some people also experience dizziness or spotting between periods. These effects are temporary and typically resolve within a few days.
Nausea is the side effect most likely to cause a practical problem. If you vomit within two hours of swallowing the pill, your body may not have absorbed enough of the hormone for it to work. In that case, you should take another dose.
What Happens to Your Next Period
Plan B frequently changes the timing of your next menstrual cycle. Your period may arrive a few days early or a few days late. It may be heavier or lighter than usual, and you might notice spotting in the days or weeks before it shows up. These changes happen because the large hormone dose temporarily disrupts your cycle’s normal rhythm, and your body needs time to recalibrate.
If your period is more than a week late, take a pregnancy test. A late period doesn’t necessarily mean Plan B failed, but it’s the simplest way to know for sure.
Weight Affects How Well It Works
This is something many people don’t realize: Plan B becomes significantly less effective at higher body weights. Research shows that the pill’s efficacy starts to decline at around 70 kg (about 154 pounds) and appears to have essentially no effect for people weighing 80 kg (about 176 pounds) or more. The reason is pharmacological. At higher body weights, peak concentrations of the drug in the blood drop by roughly 50 percent compared to people with a lower BMI, which means there may not be enough hormone circulating to reliably block ovulation.
People with a BMI of 30 or higher who used Plan B had more than four times the risk of pregnancy compared to those with a BMI under 25. If your weight falls in this range, a different form of emergency contraception will be more reliable.
Medications That Can Reduce Effectiveness
Certain medications speed up how quickly your liver breaks down the hormone in Plan B, which can cut its effectiveness. These include some drugs used to treat epilepsy, tuberculosis, HIV, and fungal infections. The HIV drug efavirenz, for example, reduces blood levels of the hormone by about 50 percent.
St. John’s wort, an herbal supplement commonly taken for mood, also lowers hormone levels through the same liver pathway. If you take any of these medications or supplements regularly, Plan B may not work well enough on its own.
Other Emergency Contraception Options
Plan B is the most widely available option, but it’s not the most effective one. Two alternatives outperform it, especially when time or body weight is a factor.
- Ella (ulipristal acetate): A prescription morning-after pill that works for up to five days after unprotected sex and maintains its effectiveness across that entire window better than Plan B does. It is also more effective than Plan B at every time point.
- Copper IUD: The most effective form of emergency contraception available. It can be inserted up to five days after unprotected sex and works equally well on day one and day five. Certain hormonal IUDs (Mirena and Liletta) also work for emergency use. The added benefit is that once placed, an IUD provides ongoing contraception for years.
For people over 70 kg, or anyone past the 72-hour mark, ella or an IUD is a stronger choice than Plan B.
Repeated Use and Future Fertility
Using Plan B does not affect your long-term fertility. The hormone clears your system quickly, and there is no evidence that repeated use causes cumulative damage to your reproductive system or makes it harder to get pregnant later. That said, it’s designed as a backup, not a primary method. It’s less effective than regular contraception, more expensive per use, and the side effects are more noticeable than what you’d experience with daily or long-acting birth control.