How emergency contraception works (and fails)
The primary way that most oral emergency contraceptive pills (ECPs) work is by delaying or temporarily preventing ovulation, which is the release of an egg from an ovary. By stopping or delaying ovulation, the pill prevents a sperm from fertilizing an egg. ECPs are not the same as the abortion pill and will not terminate an existing pregnancy.
ECPs are not 100% effective. Failure rates vary depending on the type of pill and how soon it is taken.
Key factors influencing I pill efficacy
Several factors can influence the effectiveness of emergency contraception, explaining why there is always a chance that the I pill doesn't work as expected.
Timing is critical
The sooner you take the pill after unprotected sex, the more effective it will be. Effectiveness declines over time.
Body weight considerations
Research indicates that BMI can impact ECP effectiveness. Levonorgestrel pills may be less effective for those weighing over 165 pounds. Ulipristal acetate (Ella) may also be less effective for those over 195 pounds, but is more effective than levonorgestrel in higher-weight individuals. The copper IUD is unaffected by body weight.
Ovulation status
ECPs work by delaying ovulation. If ovulation has already occurred, the pill will not be effective. Take the ECP as soon as possible, regardless of your cycle stage.
Drug interactions
Certain medications and supplements can interfere with ECP effectiveness. These include certain anticonvulsants, St. John's wort, some antibiotics/antifungals, and certain HIV medications. Consult a healthcare provider if you are taking any of these.
Vomiting
If you vomit within a few hours of taking an ECP, you may not have absorbed the medication. Contact a healthcare provider to see if you need another dose.
Comparing emergency contraceptive pills and IUDs
Different emergency contraception options have varying characteristics and effectiveness. The table below compares these methods.
| Feature | Levonorgestrel (e.g., Plan B) | Ulipristal Acetate (Ella) | Copper IUD |
|---|---|---|---|
| Availability | Over-the-counter | Prescription only | Requires a medical appointment for insertion |
| Effectiveness Window | Up to 72 hours (3 days), but effectiveness decreases over time | Up to 120 hours (5 days), maintaining higher efficacy longer | Up to 120 hours (5 days), most effective option |
| Efficacy | Can be less effective, especially if taken later in the window | More effective than levonorgestrel, especially later in the window | >99% effective, making it the most reliable emergency contraception |
| Body Weight Impact | May be less effective if you weigh more than 165 pounds | May be less effective if you weigh more than 195 pounds | No impact from body weight |
| Mechanism | Delays or prevents ovulation | Delays or prevents ovulation, even later in the cycle | Creates a toxic environment for sperm and prevents implantation |
| Ongoing Contraception | Not for regular use | Not for regular use | Can be kept for 10+ years as a highly effective form of birth control |
For the most reliable emergency contraception, the copper IUD is most effective, followed by ulipristal acetate. You can find more information from the American College of Obstetricians and Gynecologists (ACOG).
What to do if the emergency contraceptive fails
If you are concerned that your emergency contraceptive didn't work, take these steps:
- Monitor your next period: ECPs can affect period timing. If your period is more than a week late, take a pregnancy test.
- Take a pregnancy test: Home tests detect hCG. A positive result suggests the emergency contraception failed.
- Recognize early pregnancy signs: Symptoms like breast tenderness, fatigue, or nausea could indicate pregnancy or be pill side effects. If symptoms persist, take a pregnancy test.
- Consult a healthcare provider: If you have a positive pregnancy test, discuss your options with a healthcare provider. They can also test for STIs.
Conclusion
Yes, there is a chance that the I pill doesn't work, as no emergency contraceptive is 100% effective. Failure is influenced by factors like time since unprotected sex, body weight, ovulation timing, and interacting medications. While ECPs reduce pregnancy risk, their efficacy can decrease over time. For the highest effectiveness, especially with higher body weight or further from unprotected sex, the copper IUD is the most reliable option. Act as soon as possible with any method. If concerned about efficacy, take a pregnancy test and consult a healthcare provider.
Additional options and resources
Consider other regular contraceptive methods for ongoing prevention. Discuss long-term options like IUDs, implants, or pills with a healthcare provider. Health centers like Planned Parenthood offer guidance on contraceptive options.
