Plan B One-Step contains a single active ingredient: 1.5 milligrams of levonorgestrel, a synthetic form of the hormone progestin. That one ingredient, combined with a handful of standard inactive fillers, makes up the entire pill. Here’s what each component does and why it matters.
The Active Ingredient: Levonorgestrel
Levonorgestrel is a lab-made version of progestin, a hormone your body produces naturally as part of the menstrual cycle. It’s the same hormone used in many daily birth control pills, just delivered in a single, higher dose. At 1.5 mg, this one-time dose is enough to temporarily disrupt the hormonal signals that trigger ovulation.
This is worth understanding clearly: levonorgestrel works by preventing or delaying ovulation, the release of an egg from the ovary. If no egg is released, sperm has nothing to fertilize. The FDA reviewed the science on this and concluded there is no direct effect on fertilization or implantation. The agency updated Plan B’s consumer information to remove older references to those mechanisms because they weren’t supported by the best available evidence. In plain terms, Plan B prevents pregnancy by stopping ovulation before it happens. It does not terminate an existing pregnancy.
The Inactive Ingredients
Beyond levonorgestrel, Plan B contains six inactive ingredients. These don’t have any hormonal or contraceptive effect. They’re standard pharmaceutical fillers that give the pill its shape, help it dissolve properly, and keep it stable on the shelf:
- Colloidal silicon dioxide: A flow agent that prevents the powdered ingredients from clumping during manufacturing.
- Corn starch: A common binder that helps hold the tablet together.
- Lactose monohydrate: A sugar derived from milk, used as a filler to bulk up the tablet to a manageable size.
- Magnesium stearate: A lubricant that keeps the tablet from sticking to the machinery during production.
- Potato starch: A disintegrant that helps the pill break apart in your digestive system so the active ingredient can be absorbed.
- Talc: Another flow agent that improves the consistency of the tablet mixture.
None of these ingredients are unusual. You’ll find most of them in common over-the-counter medications like ibuprofen or antihistamines.
Allergens to Know About
The most relevant allergen in Plan B is lactose monohydrate. If you have a severe lactose intolerance or a true milk protein allergy, this is worth noting, though the amount of lactose in a single tablet is very small. Most people with lactose intolerance (which involves difficulty digesting larger amounts of lactose in foods) won’t notice any issue from the trace amount in a pill. The FDA label also warns against taking Plan B if you’re allergic to levonorgestrel itself, though this is rare.
Generic Versions Use the Same Formula
Several store-brand and generic emergency contraceptives are available, including Take Action, My Way, and others. These all contain the same active ingredient at the same dose: 1.5 mg of levonorgestrel. They work identically and are equally effective. The inactive ingredients may vary slightly between brands (different fillers or coatings), but the core formula is the same. The only consistent difference is price, with generics typically costing less than the Plan B brand name.
How the Ingredient Actually Works in Your Body
When you swallow Plan B, the levonorgestrel is absorbed through your digestive tract and enters your bloodstream. From there, it mimics the effect of natural progestin at a high enough level to suppress the surge of luteinizing hormone that normally triggers ovulation midway through your cycle. Without that hormonal surge, the ovary doesn’t release an egg.
This is why timing matters so much. Plan B is most effective when taken before ovulation has occurred. If you’ve already ovulated, the pill has less ability to prevent pregnancy because its primary mechanism, blocking egg release, is no longer relevant. This isn’t a limitation of the ingredients themselves but of when in your cycle you take them.
The levonorgestrel is processed by your liver and cleared from your system relatively quickly. It doesn’t accumulate or have lasting effects on your fertility. Your normal menstrual cycle typically resumes within a few weeks, though your next period may arrive a few days earlier or later than expected.