How Does Plan B One-Step Work? Mechanism of Action Explained in Plain English

Plan B One-Step contains levonorgestrel — a synthetic form of the hormone progesterone. But how exactly does a single pill taken after sex prevent pregnancy? And what about the claims that it causes abortion? Let's break down the actual science clearly.

What Is Levonorgestrel?

Levonorgestrel is a synthetic progestin — a manufactured version of progesterone, a naturally occurring female sex hormone. It has been used in hormonal birth control pills for over 35 years. In Plan B One-Step, it's delivered at a higher dose than in daily birth control pills: 1.5 mg in a single tablet, compared to the 0.03–0.15 mg found in daily progestin-only pills.

The Primary Mechanism: Blocking Ovulation

The main way Plan B One-Step prevents pregnancy is by delaying or preventing ovulation — the release of an egg from the ovary.

Here's how that works step by step:

Under normal circumstances, a surge in luteinizing hormone (LH) triggers ovulation — the ovary releases an egg that can be fertilized by sperm.

Levonorgestrel suppresses or blocks this LH surge — preventing the ovary from releasing an egg before sperm can reach it.

Without an egg available, sperm cannot fertilize anything — and pregnancy cannot occur.

This mechanism is why Plan B works best when taken before ovulation. If ovulation has already occurred, levonorgestrel's primary mechanism cannot work.

Secondary Mechanism: Thickening Cervical Mucus

Levonorgestrel also thickens the cervical mucus — the fluid produced by the cervix. Thicker cervical mucus makes it harder for sperm to move through and reach an egg, adding a secondary barrier to fertilization.

Does Plan B One-Step Cause Abortion?

No. This is one of the most important medical facts about Plan B One-Step, and the FDA addressed it directly in 2022 when it updated the product label.

Plan B One-Step works by preventing pregnancy from occurring — specifically by preventing fertilization of an egg. It does not terminate an existing pregnancy. If you are already pregnant when you take Plan B One-Step, the medication will not affect the pregnancy in any way.

Earlier versions of the Plan B label mentioned a possible effect on "implantation" (the attachment of a fertilized egg to the uterus). The FDA updated the label in 2022 to clarify that evidence does not support that Plan B affects implantation. The primary and well-established mechanism is blocking ovulation.

Why Does Timing Matter So Much?

Since Plan B One-Step works by preventing ovulation, its effectiveness depends on when in the menstrual cycle it's taken:

If taken before the LH surge (before ovulation is triggered) — it can prevent ovulation entirely, providing strong protection

If taken after the LH surge has already started — it may be less effective because ovulation may already be underway or imminent

If ovulation has already occurred — Plan B cannot reverse it or prevent fertilization (however, sperm may survive in the reproductive tract for up to 5 days, so the window for protection is wider than it might seem)

This is why Plan B works best when taken as soon as possible after unprotected sex, and why efficacy drops as time passes.

How Does ella (Ulipristal Acetate) Work Differently?

ella (ulipristal acetate) is a selective progesterone receptor modulator (SPRM). Unlike levonorgestrel, it can delay ovulation even after the LH surge has already begun. This is why ella remains effective for up to 120 hours (5 days) while Plan B's effectiveness drops significantly after 72 hours. ella also works better at higher BMIs.

Why May Plan B Be Less Effective at Higher Body Weights?

Levonorgestrel is thought to be absorbed and distributed throughout the body based on body weight and volume. At higher body weights (BMI above 26–30), the same 1.5 mg dose produces lower blood concentrations of levonorgestrel — potentially not enough to effectively suppress ovulation. This is not a flaw in Plan B design; it's a limitation of the fixed 1.5 mg dose vs. body weight differences.

Summary: How Plan B One-Step Works

Primary mechanism: Delays or prevents ovulation by blocking the LH surge

Secondary mechanism: Thickens cervical mucus to impede sperm motility

Does NOT: Terminate an existing pregnancy or affect implantation (per 2022 FDA label update)

Best taken: As soon as possible after unprotected sex, within 72 hours

Efficacy: 75–89% reduction in pregnancy risk vs. no contraception

For information on what to expect in your body after taking Plan B One-Step, see our guide on Plan B One-Step side effects: what to expect and when to call your doctor.