Plan B One-Step is an emergency contraceptive pill containing 1.5 mg of a synthetic hormone called levonorgestrel. It’s a single pill taken by mouth after unprotected sex or contraceptive failure to prevent pregnancy before it starts. It’s available over the counter in the United States without a prescription or age restriction.
How Plan B One-Step Works
Plan B works primarily by preventing or delaying ovulation, the release of an egg from the ovary. If your body hasn’t released an egg yet, sperm have nothing to fertilize, and pregnancy can’t occur. If ovulation has already happened, some researchers have hypothesized that the pill may interfere with fertilization or implantation, though studies suggest it may not actually affect these processes.
This is the critical distinction: Plan B prevents pregnancy from occurring. It cannot end an existing pregnancy. The medical community defines pregnancy as beginning when a fertilized egg implants in the uterus, and levonorgestrel does not disrupt that process once it’s happened. If you’re already pregnant when you take it, the pill will not harm the pregnancy.
How It Differs From the Abortion Pill
Plan B and medication abortion are completely different drugs that work in completely different ways. Medication abortion uses two drugs (mifepristone and misoprostol) taken after a pregnancy is confirmed and in consultation with a medical provider. Mifepristone blocks progesterone, causing an embryo to detach from the uterine wall. Misoprostol then causes the uterus to contract and empty. Plan B does none of this. It simply delays ovulation so that pregnancy never begins in the first place.
When to Take It and How Timing Affects Effectiveness
Plan B can be taken within 72 hours (three days) of unprotected sex, but it works significantly better the sooner you take it. Effectiveness drops with each passing day. Taking it as close to the event as possible gives you the best chance of preventing pregnancy.
Plan B is not meant to replace regular birth control. It’s a backup option for situations like a broken condom, a missed pill, or unprotected sex. You take the single pill with or without food, and that’s it.
Weight and Effectiveness
Body weight has a meaningful impact on how well Plan B works. Research shows the pill may not be effective for people weighing more than about 155 to 165 pounds. Clinical studies have found that levonorgestrel-based emergency contraception appears to hit a ceiling of effectiveness at around 154 pounds (70 kg) and may have no emergency contraceptive effect at all for people weighing 176 pounds (80 kg) or more. People with a BMI over 30 had more than four times the risk of pregnancy compared to those with a BMI in the normal range. Those with a BMI between 25 and 30 were about twice as likely to experience failure.
If you weigh more than about 155 pounds, a prescription alternative called ella (ulipristal acetate) is effective for people up to about 195 pounds. It can be taken within five days of unprotected sex, though it also works better the sooner you take it. A copper IUD, inserted by a healthcare provider within five days, is another highly effective option regardless of weight.
Side Effects
Side effects are mild for most people and typically last only a few days. The most common ones include:
- Nausea or vomiting
- Headache and dizziness
- Fatigue
- Breast tenderness
- Abdominal pain or cramping
- Spotting or light bleeding between periods
Your next period may come earlier or later than expected, and it may be heavier or lighter than usual. This is normal and not a sign that something is wrong. However, if your period is more than a week late, it’s worth taking a pregnancy test.
How to Get It
Plan B One-Step is sold over the counter at pharmacies, drugstores, and many grocery stores. You do not need a prescription, and there is no age requirement to purchase it. You can also find generic versions containing the same 1.5 mg dose of levonorgestrel, which tend to cost less. The pill is typically stocked on shelves near other family planning products, though some stores keep it behind the pharmacy counter (you can still buy it without a prescription in those cases). Prices generally range from about $30 to $50, depending on the brand and retailer.