What Makes ellaOne Not Work? Understanding Emergency Contraception Failure

October 10, 2025 •

3 min read

When used correctly, the emergency contraceptive pill ellaOne is highly effective at preventing pregnancy. However, a range of physiological and external factors can compromise its efficacy, leaving many to wonder, what makes ellaOne not work in certain situations? Understanding these limitations is crucial for anyone relying on emergency contraception.

Quick Summary

Several factors can cause ellaOne to fail, most critically if ovulation has already occurred before the pill is taken. Other issues include vomiting, interactions with certain medications, higher body weight, and improper timing or subsequent unprotected sex after use.

Key Points

Timing is critical: ellaOne must be taken as soon as possible, and no later than 120 hours (5 days), after unprotected sex to be most effective.
It is ineffective after ovulation: The pill's primary mechanism is delaying ovulation, so it will not work if the egg has already been released.
Body weight can reduce efficacy: Studies show that ellaOne may be less effective in individuals with a higher BMI, potentially over 30.
Drug interactions are a risk: Several medications, including certain antiepileptics, HIV drugs, and the herbal supplement St. John's Wort, can decrease ellaOne's effectiveness.
Vomiting requires a repeat dose: If you vomit within three hours of taking ellaOne, the medication may not be absorbed, and you should seek medical advice for another dose.
It offers no ongoing protection: The pill does not protect against future acts of unprotected sex in the same cycle, and fertility can return quickly.

The emergency contraceptive pill ellaOne (ulipristal acetate) primarily works by delaying or inhibiting ovulation. Since sperm can live for up to five days, delaying ovulation prevents fertilization by ensuring the sperm die before an egg is released. ellaOne is effective for up to 120 hours (five days) after unprotected sex. However, its success depends on taking it before ovulation occurs, as it is ineffective afterward. Taking it as soon as possible after unprotected sex is crucial for maximum effectiveness due to the unpredictable nature of ovulation.

Factors That Undermine ellaOne's Efficacy

Several factors can reduce ellaOne's effectiveness, including individual characteristics, interactions with other medications, and incorrect usage.

The Critical Impact of Body Weight

Research suggests that ellaOne may be less effective in individuals with a higher body mass index (BMI), particularly those with a BMI over 30 or weighing more than 195 pounds. While the exact reasons are still being studied, it's thought to relate to how hormones are absorbed and processed. The copper IUD is a highly effective, weight-independent emergency contraception option for those concerned about weight.

Drug Interactions to Be Aware Of

Certain medications can reduce the amount of ellaOne in the body, making it less effective. These include some medications for epilepsy (e.g., phenytoin), tuberculosis (e.g., rifampicin), and HIV treatments. Taking ellaOne with a levonorgestrel-based pill (like Plan B) is not advised as they can counteract each other. Starting hormonal birth control too soon after taking ellaOne can also reduce effectiveness. The herbal supplement St. John's Wort can also decrease ellaOne's efficacy.

Improper Use and Other Scenarios

If vomiting occurs within three hours of taking ellaOne, the medication might not have been fully absorbed, and medical advice should be sought for a repeat dose or alternative. ellaOne only protects against pregnancy from the single instance of unprotected sex; it does not provide protection for subsequent acts of unprotected sex in the same cycle. ellaOne is not an abortion pill and will not end an existing pregnancy.

A Comparison of Emergency Contraceptive Options


Feature	ellaOne (Ulipristal Acetate)	Levonorgestrel (Plan B One-Step)	Copper IUD (ParaGard)
Mechanism	Delays or inhibits ovulation even closer to the LH surge.	Delays or inhibits ovulation by acting as a synthetic progestin.	Creates a toxic uterine environment for sperm and eggs, preventing fertilization and implantation.
Time Window	Up to 120 hours (5 days) after unprotected sex.	Up to 72 hours (3 days) after unprotected sex, with rapidly decreasing efficacy.	Up to 120 hours (5 days) after unprotected sex, or within 5 days of ovulation.
Efficacy	98-99% effective when taken within 24 hours, remaining high throughout the 5-day window.	Up to 95% effective within 24 hours, but efficacy declines sharply afterward.	Over 99% effective, making it the most effective option.
Weight Impact	Potentially less effective for those with a BMI over 30.	Potentially less effective for those with a BMI over 25.	No impact from body weight.
Accessibility	Prescription-only in many regions.	Available over-the-counter in many regions, often with no age restrictions.	Requires a healthcare provider to insert.

What to Do After Taking ellaOne

After using ellaOne, it's important to be aware that your next period might be different than usual. If your period is more than a week late, take a pregnancy test. Since fertility returns quickly, use barrier methods for any subsequent sexual activity until your next period.

Conclusion: When in Doubt, Consult a Professional

ellaOne's effectiveness can be compromised by factors like the timing relative to ovulation, other medications, or body weight. It is a valuable emergency contraceptive but not 100% effective. Taking it promptly and discussing potential drug interactions with a healthcare provider or pharmacist are crucial for maximizing efficacy. For those with concerns or contraindications, the copper IUD is the most effective alternative. Consulting a healthcare professional is always the best way to determine the most suitable option for your individual needs.

Visit Planned Parenthood for more information about emergency contraception.

Frequently Asked Questions

Yes, while ellaOne is highly effective when taken correctly, no emergency contraceptive is 100% foolproof. A very small percentage of users may still become pregnant.

Yes, some studies suggest that ellaOne may be less effective for individuals with a higher body weight or a BMI over 30.

If you vomit within three hours of taking ellaOne, it may not have been absorbed. You should contact a doctor or pharmacist immediately, as you may need another dose or to consider an alternative, such as a copper IUD.

You should tell a healthcare professional about all other medications you are taking, including herbal remedies like St. John's Wort. Some drugs, such as certain epilepsy and HIV treatments, can reduce ellaOne's efficacy.

Taking ellaOne with a levonorgestrel-based pill could reduce the effectiveness of both medications, as they can interfere with each other's mechanisms.

No, ellaOne is not effective if ovulation has already occurred. Its primary action is to delay the release of an egg.

No, ellaOne only prevents pregnancy from the specific act of unprotected sex for which it was taken. Fertility can return immediately, so a barrier method should be used for any subsequent sexual activity.