What is Plan B and how does it work?
Plan B One-Step is a progestin-only emergency contraceptive that contains a high dose of the synthetic hormone levonorgestrel. Unlike regular birth control, it is intended for backup use and is most effective when taken as soon as possible after unprotected sexual intercourse. The pill works primarily by delaying or preventing ovulation, the release of an egg from the ovary. By doing so, it stops a sperm from fertilizing an egg. It is important to note that Plan B is not an abortion pill; it does not terminate an existing pregnancy. If implantation has already occurred (when a fertilized egg attaches to the uterine wall), Plan B will not be effective.
Factors that influence the failure rate of Plan B
Several key factors can influence the effectiveness of Plan B, and consequently, the likelihood of its failure. Understanding these variables is crucial for anyone relying on emergency contraception.
Time since unprotected sex
The most critical factor determining Plan B's success is how quickly it is taken. Research consistently shows that the sooner a person takes the pill after unprotected sex, the more effective it is. The pill is approved for use within 72 hours (3 days), but its effectiveness wanes over this period. Some sources, including the manufacturer, indicate an efficacy rate of up to 89% when taken within 72 hours. However, studies suggest a higher effectiveness when taken within the first 24 hours, with the risk of pregnancy increasing over time.
Body weight or Body Mass Index (BMI)
Some studies have indicated that Plan B may be less effective in individuals with a higher body weight or body mass index (BMI). While the FDA has stated the data is inconclusive on this matter, it's a concern frequently raised by healthcare providers. One analysis showed a decreased efficacy for women with a higher BMI, suggesting that for individuals weighing more than 165 pounds, or with a BMI over 30, alternative options like the copper IUD or ella may be more effective.
Menstrual cycle timing
Plan B works by delaying ovulation. This means if ovulation has already occurred before the pill is taken, the medication will not be effective. A study cited in Clinical Obstetrics and Gynecology found that women who took levonorgestrel on or after the day of ovulation had a similar pregnancy rate to those who used no contraception at all. However, since it can be challenging to pinpoint the exact timing of ovulation, healthcare providers typically advise taking Plan B as soon as possible regardless of where a person believes they are in their cycle.
Interaction with other medications
Certain medications and herbal supplements can interfere with the effectiveness of Plan B by speeding up the metabolism of the levonorgestrel. It is important to inform a healthcare provider of any other medications being taken. Medications that can lower Plan B's effectiveness include:
- Barbiturates
- Certain anti-seizure medications (e.g., carbamazepine, topiramate)
- The antibiotic rifampin
- The antifungal griseofulvin
- The herb St. John's wort
Plan B failure vs. other emergency contraception options
When assessing how often Plan B pills fail, it's helpful to compare its effectiveness with other emergency contraception methods. The following table provides a comparison based on timing and general efficacy.
| Method | Active Ingredient | Timing for Use | Efficacy | Notes |
|---|---|---|---|---|
| Plan B (and generics) | Levonorgestrel | Up to 72 hours (3 days) | 75–89% effective within 72 hours | Most effective when taken sooner. May be less effective for individuals with higher body weight. |
| ella (ulipristal acetate) | Ulipristal acetate | Up to 120 hours (5 days) | 85% effective within 120 hours | Prescription-only. Maintains effectiveness for the full 5-day period. Less affected by body weight compared to Plan B. |
| Copper IUD (Paragard) | Copper | Up to 120 hours (5 days) | >99% effective | Requires insertion by a healthcare provider. Most effective option available. Can be left in place for long-term contraception. |
How to proceed if Plan B may have failed
Despite the high efficacy rates of Plan B, failure can occur, leaving individuals feeling uncertain. The most definitive sign of failure is a missed period, followed by a positive pregnancy test.
- Wait for your period: Taking Plan B can sometimes alter your menstrual cycle, causing your period to arrive earlier or later than expected. If your period is delayed by a week or more, it is recommended to take a pregnancy test.
- Take a pregnancy test: A home pregnancy test is the most straightforward way to determine if Plan B was unsuccessful. For accuracy, follow the test instructions carefully and consider testing three to four weeks after taking the pill if your period has not returned.
- Contact a healthcare provider: A healthcare provider can offer confirmation with a blood test and discuss your options, especially if you have symptoms of a possible tubal (ectopic) pregnancy, which is a rare but serious medical emergency. A pregnancy that occurs despite taking Plan B is not necessarily ectopic, but a doctor should rule this out.
Conclusion: Minimizing the risk of Plan B failure
Plan B is a safe and effective emergency contraceptive, but its success is not guaranteed. The chance of failure is influenced by factors like timing, body weight, and interaction with other medications. The most crucial step for maximizing its effectiveness is to take it as soon as possible after unprotected sex, ideally within 24 hours. For individuals who are in a higher body weight category or who waited longer than 72 hours, other emergency contraception options, such as ella or a copper IUD, may offer better protection. While a valuable backup, it is not meant to replace regular, more effective contraception methods. Based on information from Planned Parenthood, it is recommended to discuss routine birth control options with a healthcare provider for ongoing pregnancy prevention.
Additional resources
For further information on contraception, including emergency options, you can consult reliable sources like the Centers for Disease Control and Prevention (CDC), Planned Parenthood, and your personal healthcare provider.
How to maximize Plan B's effectiveness
To get the most out of your emergency contraceptive, follow these guidelines:
- Act quickly: Take the pill as soon as possible after unprotected sex. The longer you wait, the higher the chance of failure.
- Check medications: Consult a pharmacist or doctor to ensure none of your other medications or supplements interact with levonorgestrel.
- Review your weight: If you have a higher body weight, discuss more effective options like ella or the copper IUD with a healthcare provider.
- Don't have repeat unprotected sex: Plan B does not offer continuous protection. If you have unprotected sex again after taking the pill, you will need to take another dose.
- Consider long-term options: Remember that Plan B is for emergencies only. Look into more reliable, long-term birth control methods for consistent prevention.
