Plan B is a single pill that you swallow with water as soon as possible after unprotected sex, ideally within 24 hours and no later than 72 hours. There’s no special preparation, no prescription needed, and no age restriction. Here’s everything you need to know to take it correctly and what to expect afterward.
How to Take the Pill
Plan B One-Step is one tablet containing 1.5 mg of levonorgestrel, a synthetic form of progesterone. You take it by mouth with water. You can take it with or without food, though eating a small snack beforehand may help if you’re prone to nausea.
That’s it. There’s no second dose to remember, no waiting period before or after eating, and no need to take it at a specific time of day. The only rule that matters is speed: the sooner you take it after unprotected sex, the better it works.
Why Timing Matters
Plan B works primarily by delaying or preventing ovulation. If your body has already released an egg, the pill is far less likely to prevent pregnancy. This is why the clock starts the moment you have unprotected sex or experience a contraceptive failure like a broken condom.
The pill is 81 to 90% effective at preventing pregnancy when taken within 72 hours, but that range reflects a steep drop-off. Taking it in the first 24 hours gives you the best odds. By 48 to 72 hours, effectiveness has declined noticeably. After 72 hours, it is not recommended as a reliable option.
Where to Get It and What It Costs
Plan B is available over the counter at most pharmacies, drugstores, and some grocery stores. You do not need a prescription or an ID to buy it, and there is no age requirement. At some pharmacies the box is kept behind the counter, so you may need to ask a pharmacist, but you won’t need to show anything or answer medical questions.
Brand-name Plan B One-Step typically costs $40 to $50. Generic versions, sold under names like My Way, Take Action, and AfterPill, can cost as little as $11. They contain the same active ingredient at the same dose and work the same way.
What to Do If You Vomit
If you throw up within two hours of swallowing the pill, your body likely didn’t absorb enough of the medication for it to work. You’ll need to take another dose. If nausea is a concern, taking the pill with a light meal or snack can help keep it down. If you vomit after the two-hour window, the drug has already been absorbed and you don’t need a second dose.
Weight and Effectiveness
Plan B becomes significantly less effective at higher body weights. Research from Oregon Health & Science University found that people with a BMI of 30 or above experienced morning-after pill failure four times as often as those with a BMI under 25. Blood levels of the active ingredient were about 50% lower in individuals with a BMI of 30 after a standard dose, meaning the drug likely never reaches the concentration needed to block ovulation. Doubling the dose does not solve this problem.
If your BMI is 30 or higher, or you weigh more than about 165 pounds, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraception option regardless of weight. Another prescription emergency contraceptive pill (sold as ella) is effective up to a higher weight threshold than Plan B, though it does require a prescription.
Common Side Effects
Most people experience mild side effects or none at all. The most common ones include nausea, headache, fatigue, dizziness, breast tenderness, and lower abdominal cramping. These typically resolve within a day or two.
Light spotting between when you take the pill and your next period is also normal. Plan B is not associated with serious or long-term side effects, and it will not affect your ability to become pregnant in the future.
What Happens to Your Period
Expect your next period to behave differently than usual. It may arrive earlier or later than expected, and the flow might be heavier, lighter, or more irregular. A period that comes a few days early or a week late is one of the most common effects of emergency contraception and is not a sign that anything is wrong.
If your period is more than a week late, take a pregnancy test. In general, a pregnancy test taken three weeks after you took Plan B will give you an accurate result. Testing earlier than that can produce a false negative because pregnancy hormone levels may not yet be high enough to detect.
Medications That Reduce Effectiveness
Certain medications can interfere with how your body processes Plan B, making it less effective. The most well-known interaction is with St. John’s wort, an herbal supplement that speeds up the breakdown of hormones in your body. Several prescription medications used for seizures, HIV, and certain other conditions can have the same effect. If you take any prescription medication regularly, mention it to a pharmacist when you pick up Plan B so they can flag any concerns.
Resuming Regular Birth Control
You can restart or begin your regular hormonal birth control (the pill, patch, or ring) right away after taking Plan B. However, you’ll need to use a backup method like condoms for the first seven days after restarting, since the emergency contraception disrupts your normal hormonal cycle and your regular method needs time to become effective again.
Plan B is not designed to replace ongoing contraception. It delivers a much higher hormone dose than daily birth control pills, and using it repeatedly is both more expensive and less reliable than a consistent method.