Plan B is a single pill you take by mouth as soon as possible after unprotected sex. There’s no special timing with food, no multi-step process, and no prescription needed. You swallow one tablet with water, and that’s it. But when you take it, how much you weigh, and what medications you’re on all affect how well it works.
When to Take It
Speed matters more than anything else with Plan B. The pill is roughly 94% effective when taken within the first 24 hours after unprotected sex. By the 72-hour mark (three days), effectiveness drops to about 58%. It can technically be used up to 120 hours (five days) after sex, but the benefit continues to decline past three days.
Plan B works by delaying or preventing ovulation. If your body has already released an egg, the pill is far less likely to help. It does not end a pregnancy that has already begun. This is why the clock starts ticking immediately: the sooner you take it, the better your chances of preventing ovulation before sperm can reach an egg.
How to Get It
Plan B One-Step is available over the counter at pharmacies, drugstores, and many retail stores with a health section. You don’t need a prescription, you don’t need to show ID, and there are no age restrictions. It’s typically shelved in the family planning or feminine care aisle. Expect to pay between $40 and $50, though generic versions of levonorgestrel emergency contraception are often cheaper.
What to Do After You Take It
If you vomit within two hours of swallowing the pill, you may need to take a second dose, because your body likely didn’t absorb enough of the medication. If nausea is a concern, eating a small snack before taking the pill can help settle your stomach.
Your next period may come earlier or later than expected. Spotting between periods is also common. These changes are temporary and caused by the hormone surge from the pill. If your period is more than a week late, take a pregnancy test. Planned Parenthood recommends testing three weeks after taking Plan B for the most reliable result.
If you’re on regular hormonal birth control and took Plan B because of a missed pill or other lapse, you can restart your regular contraception the next day. Use a backup method like condoms for at least seven days after restarting, since your regular birth control needs time to become effective again.
Weight Can Affect How Well It Works
This is something many people don’t realize: Plan B becomes significantly less effective at higher body weights. Research shows the pill starts losing effectiveness at around 155 pounds (70 kg) and may provide little to no benefit above 176 pounds (80 kg). People with a BMI of 30 or higher achieve peak hormone levels roughly 50% lower than those with a BMI under 25, which likely explains the higher failure rates.
UK clinical guidelines suggest that people at higher weights take a double dose (two pills, or 3 mg total) to compensate. If doubling the dose isn’t practical or preferable, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraception at any weight, preventing virtually 100% of expected pregnancies in clinical data. Another option is ella (ulipristal acetate), a prescription-only emergency contraceptive that maintains better effectiveness across a wider weight range and works for up to five days.
Plan B vs. Ella
Ella works through a different mechanism and has a longer effective window: it’s approved for use up to 120 hours (five days) after unprotected sex without the same steep drop in effectiveness that Plan B experiences after the first day or two. If you’re past the 48-hour mark, or if you weigh more than 155 pounds, ella is generally the better option. The tradeoff is that ella requires a prescription, which can add time and cost to the process.
One important distinction: you should not take Plan B and ella together. They can interfere with each other. Choose one or the other.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down the hormone in Plan B, leaving less of it in your bloodstream. These include medications used for epilepsy (such as carbamazepine and phenytoin), tuberculosis (rifampicin), HIV, and some antifungal drugs. The herbal supplement St. John’s Wort also reduces Plan B’s effectiveness. This interaction can persist for up to four weeks after you stop taking the interfering medication.
If you’re on any of these medications, a copper IUD is the most reliable emergency contraception option. Alternatively, doubling the Plan B dose to 3 mg (two pills) can help offset the reduced hormone levels, though this approach is less well-studied than the IUD.
How to Know If It Worked
There’s no way to tell immediately whether Plan B prevented pregnancy. The most reliable indicator is your next period arriving on schedule, or close to it. Because the pill can shift your cycle by a few days in either direction, a slight change in timing is normal and not cause for alarm.
If your period doesn’t come within three weeks of taking Plan B, take a home pregnancy test. Testing before the three-week mark can produce a false negative because pregnancy hormone levels may not yet be high enough to detect. If the test is positive, Plan B did not cause any harm to the pregnancy. It simply didn’t prevent ovulation in time.