Understand how Afterpill works in plain language — from its mechanism of action to why timing matters and common misconceptions.
Afterpill (Levonorgestrel 1.5 mg) prevents pregnancy primarily by delaying or stopping ovulation — the release of an egg from the ovary. No egg means no fertilization, and no fertilization means no pregnancy.
That's the core of it. But there's more to the story, and understanding how Afterpill works can help you feel more informed and confident about using it. Let's break it down step by step.
To understand how Afterpill works, it helps to know what happens during a typical menstrual cycle:
Emergency contraception like Afterpill targets this process at critical points to prevent pregnancy.
Afterpill contains Levonorgestrel, a synthetic form of the natural hormone progesterone (specifically, a progestin). When you take Afterpill, the high dose of Levonorgestrel sends a signal to the brain's pituitary gland to suppress the LH surge — the hormonal trigger for ovulation.
Without the LH surge, the egg stays in the ovary. No egg is released, so there's nothing for sperm to fertilize.
This is why timing is so important. Afterpill works best when taken before ovulation occurs. If ovulation has already happened, the medication is significantly less effective.
Researchers have studied whether Afterpill works through additional mechanisms beyond stopping ovulation:
Some studies suggest Levonorgestrel may alter the uterine lining (endometrium), potentially making it less receptive to implantation. However, current scientific evidence does not strongly support this as a significant mechanism at the emergency contraception dose.
There is limited evidence suggesting Levonorgestrel may affect cervical mucus or sperm transport, but this is not considered a primary mechanism.
The medical and scientific community generally agrees that Afterpill's primary and most important mechanism is the inhibition of ovulation. The International Federation of Gynecology and Obstetrics (FIGO) has stated that Levonorgestrel emergency contraceptive pills work by preventing or delaying ovulation and do not prevent implantation of a fertilized egg.
There are several important misconceptions to clear up:
Afterpill's effectiveness is directly tied to where you are in your menstrual cycle when you take it:
If you take Afterpill before the LH surge, it can effectively prevent ovulation and is most likely to prevent pregnancy. This is when the medication works best — up to 95% effective within the first 24 hours.
Once the LH surge has begun but ovulation hasn't occurred yet, Afterpill may still be able to delay or disrupt ovulation, but effectiveness decreases.
If ovulation has already occurred, Afterpill has very limited ability to prevent pregnancy. This is one reason why effectiveness drops significantly after 48–72 hours — by that point, ovulation may have already taken place.
This is also why effectiveness by timing looks like this:
Research has shown that Afterpill may be less effective for people who weigh more:
The reason isn't entirely clear, but it likely relates to how Levonorgestrel is distributed and metabolized at higher body weights — the dose may not be sufficient to fully suppress the LH surge.
If you weigh over 165 lbs, consider these alternatives:
For more on alternatives, see our guide on alternatives to Afterpill.
Ella works by blocking progesterone receptors, which delays ovulation even after the LH surge has begun. This means Ella can work later in the cycle than Afterpill and may be more effective at higher body weights. However, Ella requires a prescription, and Afterpill and Ella should not be taken together — they may interfere with each other.
The copper IUD works differently — it creates a hostile environment for sperm and can also prevent implantation. It's the most effective form of emergency contraception (over 99%) and works at any body weight. It requires insertion by a healthcare provider but also provides ongoing contraception for up to 10 years.
After you swallow the tablet:
This rapid absorption is why vomiting within 2 hours of taking Afterpill is a concern — the medication may not have been fully absorbed. If this happens, contact a healthcare provider about taking another dose.
Certain medications can reduce Afterpill's effectiveness by speeding up liver metabolism, lowering the amount of Levonorgestrel in your bloodstream. These include enzyme-inducing drugs like Carbamazepine, Phenytoin, Rifampin, and the herbal supplement St. John's Wort.
For a complete list, see our guide on Afterpill drug interactions.
Afterpill works by delaying or preventing ovulation through a high dose of Levonorgestrel. It's a safe, well-studied medication that does not cause abortion, does not affect future fertility, and has no lasting effects on your body.
The single most important factor in its effectiveness is how quickly you take it. Every hour counts.
Need to find Afterpill? Use MedFinder to locate it in stock near you. For more about the medication, see What Is Afterpill?, side effects, and how to save money.
You focus on staying healthy. We'll handle the rest.
Try Medfinder Concierge FreeMedfinder's mission is to ensure every patient gets access to the medications they need. We believe this begins with trustworthy information. Our core values guide everything we do, including the standards that shape the accuracy, transparency, and quality of our content. We’re committed to delivering information that’s evidence-based, regularly updated, and easy to understand. For more details on our editorial process, see here.
%2C%20Color%3DOriginal.png)
