Plan B, a single-dose emergency contraceptive pill, contains a high dose of the hormone levonorgestrel. Its primary mechanism is to delay or prevent ovulation, stopping the release of an egg from the ovary, so there's no egg for sperm to fertilize. While a safe and effective backup method, its effectiveness is not guaranteed and can be compromised by various factors. Dispelling myths and identifying real risks is vital for confident use.
Medications That Can Reduce Plan B's Effectiveness
Certain drugs can speed up the liver's metabolism of levonorgestrel, the active ingredient in Plan B, reducing its concentration in the body and lowering its efficacy. Enzyme-inducing drugs, which increase the activity of liver enzymes, are the primary culprits.
Anti-Seizure Medications (Anticonvulsants)
Several antiepileptic medications used to treat seizures and some other conditions are potent enzyme inducers. These drugs significantly accelerate the breakdown of levonorgestrel in the body, which can render Plan B less effective at preventing pregnancy. This category includes:
- Carbamazepine (Tegretol, Equetro)
- Phenytoin (Dilantin)
- Phenobarbital
- Primidone
- Topiramate (Topamax)
- Felbamate
- Oxcarbazepine
Anti-HIV Medications
Some antiretroviral drugs used in the treatment of HIV can interact negatively with Plan B, making it less effective. Efavirenz (Sustiva) is one such medication known to decrease the amount of levonorgestrel in the body.
Antibiotics and Antifungals
While most common antibiotics, like amoxicillin and penicillin, do not affect emergency contraception, a few specific ones do.
- Rifampin: A potent antibiotic used to treat tuberculosis, rifampin is a known enzyme inducer that can decrease Plan B's effectiveness. Other rifamycin antibiotics, such as rifabutin, also have this effect.
- Griseofulvin: An antifungal medication, griseofulvin can also interfere with Plan B's hormone metabolism.
Herbal Supplements
St. John's wort is an over-the-counter herbal supplement often used for depression, and it is a known enzyme inducer that reduces the effectiveness of many hormonal medications, including Plan B.
Non-Medication Factors Affecting Plan B
Beyond drug interactions, several other factors influence Plan B's efficacy. Awareness of these can help maximize its chances of success.
Timing
The effectiveness of Plan B is highly dependent on how quickly it is taken after unprotected sex. The sooner it is taken, the better it works, with optimal effectiveness within 72 hours. Taking the pill closer to the 72-hour mark provides less protection, and it is generally considered ineffective after this point.
Body Weight
Research suggests that Plan B containing levonorgestrel may be less effective in individuals with a higher body mass index (BMI), though there is no official FDA-designated weight limit. For those with a BMI over 25 (or potentially over 30), alternatives like ulipristal acetate (Ella) or a copper IUD are more effective options for emergency contraception.
Vomiting
If you vomit within two hours of taking Plan B, the medication may not have had enough time to be absorbed by your system. In this case, it is crucial to contact a healthcare provider or pharmacist to discuss whether to take another dose.
Dispelling Common Myths
It's important to clarify what does not affect Plan B's efficacy, as misinformation can lead to unnecessary anxiety.
- Alcohol: Contrary to popular belief, drinking alcohol does not make Plan B less effective. The primary concern with alcohol consumption is if it leads to vomiting within the critical two-hour absorption window. Excessive drinking can also impair judgment, which could affect the responsible use of contraception.
- Other common antibiotics: As mentioned, most antibiotics, including common ones like amoxicillin, do not interfere with Plan B.
Comparison of Emergency Contraception Options
| Feature | Plan B (Levonorgestrel) | Ella (Ulipristal Acetate) | Copper IUD |
|---|---|---|---|
| Availability | Over-the-counter (no prescription or ID needed) | Prescription required | Inserted by a healthcare provider |
| Time Frame | Most effective within 72 hours (3 days), but can be taken up to 120 hours (5 days) | Effective up to 120 hours (5 days) | Effective up to 120 hours (5 days) |
| Body Weight | May be less effective for individuals with a higher BMI | Remains effective for people with higher body weight (up to 195 lbs) | Not affected by body weight |
| Effectiveness | Up to 89% effective when taken within 72 hours | Retains high effectiveness for up to 120 hours | Nearly 100% effective |
| Mechanism | Delays or prevents ovulation | Delays or prevents ovulation | Prevents fertilization and implantation |
| Ongoing Contraception | Does not provide ongoing protection | Does not provide ongoing protection | Provides ongoing contraception for years |
Conclusion
Understanding what can undermine the effectiveness of emergency contraception is crucial for making informed decisions about reproductive health. While nothing can truly "cancel" out Plan B, specific medications, body weight, and vomiting can significantly reduce its efficacy. For those taking enzyme-inducing drugs, who have a higher BMI, or who need a longer window of protection, considering alternatives like Ella or the copper IUD is essential. The most important takeaway is that Plan B is time-sensitive—the sooner it's taken, the more effective it is. By knowing these factors and consulting a healthcare provider, individuals can ensure they choose the best and most effective course of action for their circumstances. For more information, resources like Planned Parenthood can provide valuable guidance on all forms of contraception.
