Plan B is a single pill you swallow with water, and it works best the sooner you take it after unprotected sex. Within the first 24 hours, it prevents about 95% of pregnancies. Effectiveness drops steadily after that, so timing matters more than anything else.
How to Take It
Plan B is one tablet containing 1.5 mg of levonorgestrel, a synthetic hormone. You swallow it whole with water. You can take it with or without food, though eating a small snack beforehand may help reduce nausea. There’s no special preparation, no prescription needed, and no follow-up dose required.
If you vomit within 2 hours of taking the pill, your body likely didn’t absorb enough of the medication. You’ll need to take another dose. If nausea is a concern, taking the pill with food or after an anti-nausea medication can help keep it down.
Why Timing Is Critical
Plan B works by delaying or preventing ovulation. It delivers a burst of hormone that blocks the surge your body uses to release an egg. If ovulation hasn’t happened yet, the pill can stop it. If it already has, the pill is significantly less effective.
The 72-hour window (three days) is the outer limit, but that doesn’t mean all 72 hours are equal. At 24 hours, effectiveness sits around 95%. It declines from there, so taking it the same day is always better than waiting until the next morning. Don’t wait to see if you “need” it.
Where to Get It
Plan B One-Step and its generic versions are available over the counter at pharmacies, drugstores, and many grocery stores with a pharmacy section. The FDA approved it for nonprescription use without age restrictions in 2013, meaning anyone can buy it regardless of age and without showing ID. You can also order it online, though shipping time works against the urgency of the situation. Some pharmacies keep it behind the counter or in a locked case, but you do not need to ask a pharmacist for permission to buy it.
Body Weight and Effectiveness
Plan B becomes less reliable 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. The reason is straightforward: blood levels of the active hormone were about 50% 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 fix this problem.
If your BMI is 30 or higher, a different emergency contraceptive called ella (ulipristal acetate) is more effective across a wider weight range, though it requires a prescription. A copper IUD, inserted within five days, is the most effective emergency contraceptive at any weight.
What to Expect Afterward
Common side effects include nausea, fatigue, headache, and abdominal cramping. Some people experience spotting or bleeding between periods. These effects are temporary and typically resolve within a day or two.
Your next period may arrive earlier or later than expected. The pill can shift your cycle by up to a week in either direction. If your period hasn’t arrived within three weeks of taking Plan B, take a pregnancy test. A late period alone doesn’t mean the pill failed, but three weeks is the threshold where testing makes sense.
Taking It More Than Once
You can take Plan B more than once in the same cycle if needed, and doing so is safe. It won’t reduce the pill’s effectiveness the second time, and it doesn’t cause long-term side effects. That said, using it frequently can make your periods irregular and harder to predict, and side effects like nausea may feel more disruptive when they keep recurring. It’s not designed as a regular birth control method, simply because it’s less effective than ongoing contraception and costs more per use.
Starting or Resuming Birth Control After
You can start or restart any regular birth control method (the pill, patch, ring, shot, or IUD) immediately after taking Plan B. There’s no waiting period. However, the CDC recommends using condoms or abstaining for 7 days after starting or restarting hormonal birth control, since the new method needs that time to become effective.
Breastfeeding
Plan B is safe while breastfeeding. Studies show negligible levels of the hormone transfer into breast milk, and no side effects have been reported in breastfed infants. You do not need to pump and dump afterward. The one timing note: it’s recommended for use starting 21 days after giving birth, which aligns with when fertility can return postpartum.
What Plan B Does Not Do
Plan B prevents pregnancy. It does not end an existing pregnancy. According to FDA review, the drug works by interfering with ovulation, and the totality of clinical evidence shows it does not affect implantation of a fertilized egg. If you’re already pregnant, taking Plan B will not harm the pregnancy. It also does not protect against sexually transmitted infections.