Plan B is an emergency contraceptive pill you take after unprotected sex to prevent pregnancy. It contains a synthetic hormone called levonorgestrel, which works primarily by delaying or blocking ovulation so that sperm and egg never meet. It’s available over the counter in the United States without a prescription or age restriction, and it’s most effective the sooner you take it, with about 94% effectiveness within the first 24 hours dropping to roughly 58% by 72 hours.
How Plan B Prevents Pregnancy
Plan B delivers a concentrated dose of the same type of hormone found in many daily birth control pills. Its main job is stopping or delaying the release of an egg from the ovary. If ovulation hasn’t happened yet, the hormone essentially puts it on pause so there’s no egg available to be fertilized. Research also suggests it thickens cervical mucus, which physically blocks sperm from reaching an egg.
One important distinction: Plan B does not end an existing pregnancy. A review published in the journal Contraception found that when levonorgestrel is taken after ovulation has already occurred, conception rates are similar to what you’d expect with no intervention at all. The pill has no effect on a fertilized egg that has already implanted, and there is no evidence it affects fetal development, miscarriage rates, or stillbirth.
Plan B Is Not the Abortion Pill
This is one of the most common points of confusion. Plan B prevents pregnancy before it starts. The abortion pill (mifepristone, sometimes combined with a second medication) terminates an established pregnancy by blocking progesterone, the hormone needed to maintain a pregnancy. These are completely different drugs with different purposes, different mechanisms, and different timelines. Plan B is taken within days of sex. Medication abortion is used weeks into a confirmed pregnancy.
Timing and Effectiveness
Speed matters more than almost anything else with Plan B. According to data from the Cleveland Clinic, effectiveness breaks down roughly like this:
- Within 24 hours: about 94% effective
- Within 72 hours (3 days): about 58% effective
After 72 hours, levonorgestrel-based pills like Plan B are not recommended. If more time has passed, a different emergency contraceptive containing ulipristal acetate can work up to 120 hours (five days) after sex with higher effectiveness, up to 98% in the first 24 hours. That option does require a prescription.
A copper IUD, inserted by a healthcare provider within five days, is the most effective form of emergency contraception overall and then doubles as long-term birth control.
Weight Can Affect How Well It Works
Plan B becomes significantly less effective at higher body weights. Research shows a steep increase in pregnancy risk starting around 154 to 165 pounds (70 to 75 kg), reaching a failure rate of about 6% or higher around 176 pounds (80 kg). For comparison, the failure rate in women weighing under 143 pounds (65 kg) is roughly 1.4%.
BMI tells a similar story, with effectiveness dropping sharply around a BMI of 26. If you weigh more than about 165 pounds, ulipristal acetate or a copper IUD are generally more reliable options. Plan B still has some effect at higher weights, but the reduction is meaningful enough to factor into your decision.
Common Side Effects
Most side effects are mild and last only a few days. You might experience nausea, fatigue, headache, dizziness, breast tenderness, or stomach cramps. Some people notice light spotting between periods or heavier bleeding during their next period.
The most noticeable effect for many people is a shift in their menstrual cycle. Plan B can delay your next period by up to a week. If your period is more than three weeks late after taking it, a pregnancy test is a good idea.
Repeated Use and Long-Term Safety
You can take Plan B more than once, even within the same menstrual cycle. The American College of Obstetricians and Gynecologists confirms that repeated use of levonorgestrel emergency contraception is safe. Studies have found no increased risk of serious side effects with multiple doses, though you may experience more menstrual irregularity. Plan B does not affect your future fertility. It’s simply not designed to be a primary method of birth control, because it’s less effective than daily pills, IUDs, or implants, and the side effects add up if you’re relying on it frequently.
For people who are breastfeeding, levonorgestrel passes into breast milk in minimal quantities. Studies have found no difference in breastfeeding outcomes between women who took Plan B and those who didn’t.
Medications That Reduce Effectiveness
Certain medications speed up how quickly your body breaks down levonorgestrel, which can make Plan B less effective or even ineffective. These include some drugs used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis medications like rifampicin, certain HIV treatments including efavirenz (which cuts levonorgestrel levels in the blood by about 50%), and some antifungal medications. St. John’s wort, a common herbal supplement, also reduces levonorgestrel levels. This effect can persist for up to four weeks after stopping the interfering medication, so it’s relevant even if you’ve recently discontinued one of these drugs.
How to Get It
Plan B One-Step and its generic versions are sold over the counter at pharmacies, and in many stores alongside other reproductive health products. Since 2013, there have been no age restrictions on purchasing it in the United States, and no ID is required. There are currently 11 approved generic versions, all with the same active ingredient and the same nonprescription status. Prices typically range from $10 to $50, with generics on the lower end.