How Does the Morning After Pill Work? Mechanism of Action Explained in Plain English

One of the most misunderstood medications in pharmacies is the morning after pill. What does it actually do inside your body? Why does it work better the sooner you take it? And how are Plan B and ella different? This guide answers all of those questions in plain language—no medical degree required.

The Basics: How Does Pregnancy Happen?

To understand how the morning after pill works, it helps to understand how pregnancy begins. During the menstrual cycle, the ovary releases an egg (a process called ovulation). If sperm is present in the reproductive tract when ovulation occurs, the egg can be fertilized. The fertilized egg then travels to the uterus and implants in the uterine lining—at which point pregnancy is established.

Sperm can survive in the reproductive tract for up to 5 days. This means that even if you have sex before you ovulate, sperm can still be waiting when ovulation occurs a few days later—and pregnancy can result.

How Plan B (Levonorgestrel) Works

Plan B (levonorgestrel 1.5 mg) is a synthetic version of progesterone—a hormone your body naturally produces. When taken in a high dose, levonorgestrel works mainly by:

1. Delaying or preventing ovulation. This is the primary mechanism. By suppressing the hormonal signals that trigger the ovary to release an egg, Plan B essentially hits the pause button on ovulation. With no egg released, fertilization cannot occur, and pregnancy is prevented.

Plan B may also affect the cervical mucus, making it thicker so sperm have a harder time traveling to the egg. There is also some research suggesting it may affect the uterine lining, but this secondary effect is much less significant than ovulation suppression.

Critically: Plan B does NOT work if you have already ovulated when you take it, and it does NOT end a pregnancy that has already been established.

Why Timing Matters So Much for Plan B

Because Plan B works primarily by delaying ovulation, its effectiveness depends heavily on where you are in your menstrual cycle when you take it. If you take it before ovulation occurs—when the pill can successfully delay or prevent that egg release—it is up to 94% effective. If ovulation has already happened, Plan B cannot undo it, and it is much less likely to prevent pregnancy.

Since you can't easily know exactly when you are about to ovulate, taking Plan B as quickly as possible after unprotected sex maximizes the chance it can suppress that ovulation before it happens.

How Ella (Ulipristal Acetate) Works Differently

Ella (ulipristal acetate 30 mg) works by a different mechanism. It is a selective progesterone receptor modulator—meaning it blocks the receptors in your body that progesterone binds to. This interference disrupts the hormonal cascade that normally leads to ovulation.

One key advantage of ella's mechanism is that it can delay ovulation even after the body's pre-ovulation hormonal surge (the LH surge) has already begun—a point at which Plan B becomes ineffective. This is why ella maintains better efficacy further into the 5-day window and why it is consistently more effective than Plan B.

Like Plan B, ella does not work if ovulation has already occurred, and it does not terminate an established pregnancy.

The Simple Summary: What Emergency Contraception Does and Does Not Do

Here is a quick table for clarity:

• DOES: Delay or prevent ovulation
• DOES: Reduce the chance of pregnancy after unprotected sex
• DOES NOT: End an established pregnancy
• DOES NOT: Protect against sexually transmitted infections
• DOES NOT: Replace ongoing contraception methods

How the Copper IUD Works as Emergency Contraception

For completeness, the copper IUD (Paragard) works through a different mechanism than hormonal EC pills. Copper is toxic to sperm. When placed in the uterus, the copper IUD creates an environment hostile to sperm, preventing fertilization. It may also affect the uterine lining to prevent implantation. This mechanism works regardless of where a person is in their cycle—making it effective even if ovulation has already occurred. That is why it is more than 99% effective as emergency contraception, at any point within 5 days.

Want a full guide to what the morning after pill is and how to get it? Read what is the morning after pill. Need to find a pharmacy with it in stock near you? medfinder can help.