Plan B, or levonorgestrel, is a highly effective form of emergency contraception when taken correctly. However, its efficacy is not absolute and can be significantly reduced by interactions with certain medications, herbal supplements, and other physiological factors. This is particularly important because the medication works by preventing or delaying ovulation, and any factor that lowers the concentration of the hormone in the body can reduce its ability to do so.
Medications That Interfere with Plan B
Plan B's effectiveness is primarily affected by drugs that speed up the liver's metabolism of the hormone levonorgestrel. The primary way this happens is through the induction of the CYP3A4 enzyme, which breaks down the active ingredient faster than usual.
Enzyme-Inducing Drugs
These medications increase the activity of liver enzymes, causing the body to process levonorgestrel more quickly. This leaves less of the active hormone in your system and for a shorter amount of time, reducing its contraceptive effect.
- Rifampin: An antibiotic used to treat tuberculosis and other infections. This is a well-known enzyme inducer.
- Griseofulvin: An antifungal medication.
- Certain Anticonvulsants: A class of seizure medications that includes carbamazepine, phenytoin, phenobarbital, and primidone.
- HIV Medications: Some antiretroviral drugs, particularly certain protease inhibitors and non-nucleoside reverse transcriptase inhibitors, can interact with Plan B.
- Bosentan: A medication used to treat pulmonary hypertension.
Herbal Supplements to Avoid
While often perceived as harmless, some herbal products can have potent drug interactions. St. John's wort is the most notable example.
- St. John's Wort: This herbal supplement, commonly used for depression, is a powerful inducer of the same liver enzymes that break down levonorgestrel. The interaction can dramatically reduce Plan B's effectiveness.
Other Factors Affecting Plan B's Efficacy
Beyond drug interactions, several other circumstances can compromise the success of emergency contraception.
Vomiting
If you vomit within two hours of taking Plan B, the medication may not have been fully absorbed by your body. In this case, you will need to take another dose to ensure its effectiveness. To prevent a repeat episode, anti-nausea medication can be taken beforehand, though it is always best to consult with a healthcare provider first.
Body Mass Index (BMI)
Evidence suggests that Plan B and other levonorgestrel-based pills may be less effective for people with a higher body weight or BMI. While the exact mechanism is not fully understood, some studies indicate a higher BMI can affect how well the hormone works or is absorbed. For individuals with a BMI over 30, alternatives like ella or the copper IUD are often recommended as more reliable options. Doubling the dose of Plan B is not proven to increase its effectiveness for people with a high BMI.
Timing
Emergency contraception is most effective when taken as soon as possible after unprotected sex. The effectiveness of Plan B, specifically, decreases the longer a person waits. It should be taken within 72 hours (3 days), but its effectiveness is highest in the first 24 hours. Delaying the dose gives ovulation a greater chance of occurring.
Comparison of Emergency Contraception Options
| Feature | Plan B (Levonorgestrel) | ella (Ulipristal Acetate) | Copper IUD |
|---|---|---|---|
| Timing Window | Up to 72 hours (3 days) after unprotected sex. | Up to 120 hours (5 days) after unprotected sex. | Up to 120 hours (5 days) after unprotected sex. |
| Mechanism | Delays or prevents ovulation. | Delays or prevents ovulation; inhibits implantation. | Prevents fertilization and implantation. |
| Weight Effectiveness | Potentially less effective for those with a high BMI (>25-30). | Less affected by higher weight than Plan B. | Not affected by weight. |
| Drug Interactions | Reduced efficacy with enzyme-inducing drugs and St. John's wort. | Efficacy may be reduced by enzyme-inducing drugs, but not recommended for use with progestin-containing birth control. | Not affected by drug interactions. |
| Prescription Needed? | No prescription needed in the US. | Prescription needed. | Prescription and clinical insertion required. |
| Effectiveness Rate | Decreases over time within 72 hours. | Consistent high effectiveness throughout 120 hours. | Most effective form of EC (>99%). |
Conclusion
While a variety of factors can weaken the effectiveness of Plan B, no medication truly 'cancels it out' in a definitive way; rather, these interactions significantly reduce its efficacy, increasing the risk of pregnancy. The most critical risk factors involve medications and supplements that induce liver enzymes, such as rifampin and St. John's wort. Additionally, vomiting within two hours requires a repeat dose, and higher body weight may decrease the pill's effectiveness. Always inform your healthcare provider of all medications and supplements you are taking. If you are concerned about potential drug interactions or have a higher BMI, discuss alternative emergency contraception options like ella or the copper IUD with a medical professional. The sooner emergency contraception is used, the more effective it is, regardless of the method. For more detailed information on emergency contraception, you can consult sources like Planned Parenthood.
Planned Parenthood: The Morning-After Pill
What to Do If Emergency Contraception Fails
If you have taken Plan B and have a pregnancy concern, the most important step is to take a pregnancy test at the appropriate time, typically a few weeks after the potential conception. A positive test means that the emergency contraception was not successful. At that point, you should consult a healthcare provider to discuss your options. Medical professionals can provide guidance on prenatal care or other choices available to you.
Key Considerations for Using Plan B
- Enzyme-Inducing Medications: Anticonvulsants like carbamazepine, the antibiotic rifampin, and some HIV drugs can reduce Plan B's effectiveness by speeding up metabolism.
- St. John's Wort: This common herbal supplement is a powerful enzyme inducer and should not be used with Plan B, as it can significantly lower its efficacy.
- Vomiting within 2 hours: If you throw up within two hours of taking Plan B, the dose likely was not fully absorbed and needs to be repeated.
- Higher BMI: Recent evidence suggests that Plan B may be less effective for individuals with a higher BMI, with alternatives like ella or the copper IUD offering more consistent results.
- Alternative EC Options: If you take interacting medications or have a higher BMI, discuss other emergency contraceptive options, such as ella or the copper IUD, with a healthcare provider.
- Timing is Critical: The sooner Plan B is taken after unprotected sex, the more effective it is. Efficacy decreases significantly the longer you wait.
When to Contact a Healthcare Professional
If you have concerns about Plan B's effectiveness, a missed period, or potential side effects, contact a healthcare provider. They can provide personalized medical advice and help you navigate your options, including further testing or alternative contraceptive methods.
