Plan B One-Step contains a single active ingredient: 1.5 milligrams of levonorgestrel, a synthetic form of progesterone. It’s the same hormone used in many daily birth control pills, just at a higher dose designed to work as emergency contraception in one tablet. Beyond that active ingredient, the pill contains a handful of inactive fillers and binders that hold the tablet together.
The Active Ingredient: Levonorgestrel
Levonorgestrel is a progestin, meaning it mimics the natural hormone progesterone that your body already produces during your menstrual cycle. In daily birth control pills, levonorgestrel is delivered in small doses over weeks. In Plan B, the entire 1.5 mg dose comes in a single pill, creating a strong hormonal signal that disrupts the events leading to pregnancy.
The FDA has determined that Plan B works by stopping or delaying the release of an egg from the ovary. That surge of synthetic progesterone essentially puts ovulation on pause. If no egg is released, sperm have nothing to fertilize. The FDA has also stated that the evidence supports “no direct effect on postovulatory processes, such as fertilization or implantation,” meaning Plan B does not prevent a fertilized egg from attaching to the uterus.
Inactive Ingredients
The rest of what’s in the tablet is standard pharmaceutical filler. According to the FDA-approved label, the inactive ingredients are:
- Colloidal silicon dioxide
- Corn starch
- Potato starch
- Magnesium stearate
- Talc
- Lactose monohydrate
Most of these are unremarkable, but lactose monohydrate is worth noting if you have a severe lactose intolerance or dairy allergy. The amount is small, and most lactose-intolerant people tolerate it fine in tablet form, but it’s good to be aware of.
How Effective It Is, and When
Plan B is 81 to 90 percent effective at preventing pregnancy, with that range depending almost entirely on timing. It works best when taken as soon as possible after unprotected sex, ideally within the first 24 hours. You can take it up to 72 hours (3 days) afterward, and it can still have some effect between 73 and 120 hours (3 to 5 days), though effectiveness drops considerably in that window.
Body weight also plays a role. Research has found that Plan B appears less likely to work for women with a BMI above 26. If that applies to you, a different emergency contraceptive called ella (which contains ulipristal acetate, a progesterone modulator rather than a progestin) may be a better option. Ella requires a prescription but remains effective at higher body weights and works up to 5 days after unprotected sex with less drop-off in efficacy over time.
Common Side Effects
Because levonorgestrel is a concentrated hormone dose, side effects are common but typically mild and short-lived. The most frequently reported ones include nausea, headache, fatigue, dizziness, breast tenderness, lower abdominal pain, and changes to your next period. Your period may come earlier or later than expected, and the flow might be heavier or lighter than usual.
Vomiting is possible. If you throw up within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose. These side effects generally resolve on their own within a day or two.
What Can Reduce Its Effectiveness
Certain medications can speed up how quickly your liver breaks down levonorgestrel, leaving less of it in your system to do its job. Some anti-seizure medications and the herbal supplement St. John’s Wort are known to have this effect. If you take any medications regularly, it’s worth checking whether they interact with levonorgestrel before relying on Plan B.
How Plan B Differs From Ella
Plan B and ella are both emergency contraceptives, but they contain different active ingredients that work in slightly different ways. Plan B’s levonorgestrel is a progestin, the same type of hormone found in standard birth control pills. Ella’s active ingredient, ulipristal acetate, is a progesterone modulator, which means it blocks progesterone’s effects at its receptors rather than mimicking the hormone. This distinction matters practically: ella maintains its effectiveness longer (up to 5 full days) and works better for people with a higher BMI. Plan B is available over the counter without a prescription, while ella requires one.