Plan B, containing the hormone levonorgestrel, is a highly effective form of emergency contraception designed to prevent pregnancy after unprotected sex. It works primarily by delaying or preventing ovulation, the release of an egg from the ovary, so that sperm cannot fertilize it. It is important to note that Plan B is not an abortion pill and will not terminate a pregnancy that has already been established. For most people, when taken correctly, Plan B significantly lowers the risk of pregnancy. However, it is not infallible, and several factors can reduce its effectiveness.
The Real-World Effectiveness and Failure Rates of Plan B
Plan B's effectiveness is strongly linked to how soon it is taken after unprotected sex. The sooner the better. When taken within 24 hours, studies have shown that levonorgestrel-based emergency contraception can be up to 95% effective. Its effectiveness begins to decline with each passing day, but it can be taken up to 72 hours (three days) after the event. After 72 hours, its efficacy is significantly diminished, though some sources suggest it may still offer some protection for up to 120 hours (five days). The manufacturer's data indicates that, for those who use it as directed within 72 hours, the average risk of pregnancy is reduced from about 8% to about 1%.
This leaves a small but real chance of failure. The failure rate is estimated to be between 0.6% and 2.6% for women who take Plan B within the recommended 72-hour window. Several factors contribute to these chances, making it vital to understand the variables at play.
Why Plan B Might Fail: Factors That Reduce Efficacy
Despite its high effectiveness, there are several key reasons why Plan B may not work. Recognizing these can help an individual understand their personal risk level following an instance of unprotected sex.
Timing and Ovulation
- Taking it too late: As stated, the longer you wait to take Plan B, the less effective it becomes. Waiting beyond the 72-hour window dramatically increases the chances of conception.
- Ovulation has already occurred: Plan B works by preventing or delaying ovulation. If you have already ovulated, the egg has been released and is available for fertilization. If this is the case, Plan B is unlikely to be effective. This is one of the most common reasons for treatment failure.
Body Mass Index (BMI)
- Higher body weight: Research has suggested that levonorgestrel-based emergency contraceptives, like Plan B, may be less effective in individuals with a higher BMI, potentially over 165 pounds or a BMI of 30. For those in this category, a prescription emergency contraceptive like Ella (ulipristal acetate) or a copper IUD may offer a more effective option.
Other Influencing Factors
- Vomiting: If you vomit within two hours of taking Plan B, your body may not have absorbed the medication. In this case, you should contact a healthcare provider to see if you need to take another dose.
- Drug Interactions: Certain medications and herbal supplements can interfere with Plan B and make it less effective. Examples include St. John's Wort, certain antibiotics, and some anticonvulsant drugs.
- Expired Medication: Using a Plan B pill past its expiration date can reduce its effectiveness.
- Subsequent Unprotected Sex: Taking Plan B does not protect against future acts of unprotected sex during the same menstrual cycle. Engaging in unprotected sex again after taking the pill increases the risk of pregnancy.
Signs That Plan B Did Not Work
Early pregnancy symptoms can be very similar to the side effects of taking Plan B, which can cause confusion. The most reliable signs that Plan B failed are a missed period and a positive pregnancy test.
- Missed period: Plan B can sometimes cause a period to come a few days earlier or later than expected. However, if your period is more than a week late, you should take a pregnancy test.
- Positive pregnancy test: A positive result on a home pregnancy test is the most definitive sign that Plan B was not effective. Tests are most accurate when taken after a missed period.
- Severe lower abdominal pain: While mild cramping can be a side effect, severe or persistent pain in the lower abdomen could be a sign of an ectopic pregnancy (when a fertilized egg implants outside the uterus), which is a medical emergency.
What to Do If You Suspect Plan B Failed
If you have reason to believe Plan B did not work, it is important to take action promptly.
- Take a pregnancy test: If your period is more than a week late or you're experiencing severe symptoms, a pregnancy test is the next step.
- Consult a healthcare provider: If the test is positive, or if you have concerns, speak with a doctor or visit a clinic like Planned Parenthood to discuss your options.
- Explore alternatives and next steps: If the pregnancy is unintended, a healthcare provider can discuss all available options, including parenting, adoption, or abortion. Plan B does not harm an existing pregnancy, so there is no harm in having taken it.
Comparison of Emergency Contraception Options
| Feature | Plan B (Levonorgestrel) | Ella (Ulipristal Acetate) | Copper IUD (ParaGard) |
|---|---|---|---|
| Availability | Over-the-counter | Prescription required | Inserted by healthcare provider |
| Effectiveness Window | Up to 72 hours (3 days) | Up to 120 hours (5 days) | Up to 120 hours (5 days) |
| Effectiveness Rate | 75–89%, highest within 24 hours | About 95% effective within 120 hours | >99% effective |
| Weight Consideration | May be less effective for individuals over 165 lbs or with BMI over 30 | Works well regardless of weight up to 195 lbs or BMI 35 | Highly effective regardless of weight |
| Primary Mechanism | Delays or prevents ovulation | Delays or prevents ovulation more effectively later in cycle | Interferes with sperm mobility and prevents implantation |
| Long-Term Protection | None, only a one-time emergency solution | None, only a one-time emergency solution | Continuous birth control for up to 10-12 years |
Conclusion
While Plan B is a safe and effective emergency contraceptive, it is not 100% effective and several factors can influence the chances of it not working. Timing is the most critical factor, with effectiveness decreasing significantly after the first 24 hours. Other factors, such as body mass index, vomiting after taking the pill, and interactions with other medications, can also play a role in its failure. The only reliable signs that Plan B has failed are a missed period and a positive pregnancy test. If you suspect Plan B has not worked, the first step is to take a pregnancy test and then consult with a healthcare professional to discuss all your options. For those who find themselves frequently needing emergency contraception, discussing a more reliable long-term birth control method with a healthcare provider is a wise course of action.
For more information on emergency contraception and sexual health, visit the Planned Parenthood website.
