Plan B One-Step is an emergency contraceptive pill designed to prevent pregnancy after unprotected intercourse or contraceptive failure [1.2.2]. Its active ingredient is levonorgestrel, a hormone that primarily works by delaying or preventing ovulation [1.6.5]. When taken as directed within 72 hours, its effectiveness is high, but this can be compromised by certain external factors [1.4.5]. Understanding these factors is key to ensuring the medication works as intended.
The Critical Importance of Avoiding Interactions
Certain drugs and herbal supplements can decrease the effectiveness of Plan B [1.4.5]. They do this by inducing, or speeding up, the activity of specific liver enzymes (like CYP3A4) that are responsible for breaking down levonorgestrel in the body [1.2.4, 1.2.6]. When Plan B is metabolized too quickly, its concentration in the bloodstream drops, making it less effective at preventing ovulation and pregnancy [1.9.1]. It is crucial to review any current medications with a healthcare provider or pharmacist before taking Plan B [1.4.5].
Medications That Can Reduce Plan B's Effectiveness
Several classes of prescription and over-the-counter products are known to interact with Plan B. If you are taking any of these, you should consult a healthcare professional, who might suggest an alternative form of emergency contraception, like a copper IUD [1.9.1].
Prescription Medications
The following medications are known to decrease the plasma concentrations of levonorgestrel, potentially reducing its efficacy:
- Certain Anticonvulsants (Seizure Medications): Drugs like topiramate (Topamax), carbamazepine (Tegretol), phenytoin (Dilantin), phenobarbital, primidone, oxcarbazepine, and felbamate can all accelerate the breakdown of Plan B [1.2.1, 1.9.1]. While some experts believe lower doses of topiramate (200 mg or less) may not have an effect, the manufacturer still warns of a possible interaction [1.2.2].
- Tuberculosis Medication: Rifampin is an antibiotic well-known for causing numerous drug interactions, including with Plan B. It significantly speeds up the metabolism of levonorgestrel [1.2.2, 1.9.2].
- HIV/AIDS Medications: Certain antiretrovirals, such as efavirenz (Sustiva), can lower the amount of Plan B in your body, making it less effective [1.9.1, 1.9.5].
- Antifungals: The antifungal medication griseofulvin is listed as a substance that can decrease the effectiveness of Plan B [1.9.2, 1.9.5]. Other common antifungals are generally considered fine [1.9.2].
- Other Medications: Bosentan (a pulmonary hypertension medication) is also known to interact [1.2.4, 1.9.5].
Herbal Supplements: A Major Concern
The most widely cited herbal supplement to avoid is St. John's Wort. This popular over-the-counter remedy for depression is a potent inducer of the liver enzymes that metabolize levonorgestrel [1.2.2]. Taking St. John's Wort can cause Plan B to be cleared from the body much faster than normal, significantly reducing its ability to prevent pregnancy [1.8.3, 1.9.1].
Debunking Myths: What You Don't Need to Avoid
There are many misconceptions about what interferes with Plan B. The following do not directly reduce the pill's effectiveness:
- Most Common Antibiotics: With the exception of rifampin, most common antibiotics do not make Plan B less effective [1.9.2, 1.9.4].
- Alcohol: Alcohol does not have a direct pharmacological interaction with Plan B. However, heavy drinking that leads to vomiting within two hours of taking the pill can prevent its absorption, in which case a healthcare provider should be consulted about repeating the dose [1.2.1, 1.4.5].
- Food: You can take Plan B with or without food [1.4.5].
Emergency Contraception Comparison
If you are taking a medication that interacts with Plan B, other options are available. The copper IUD is often recommended as it is the most effective form of emergency contraception and is not affected by these medications [1.2.2, 1.7.5].
| Feature | Plan B (Levonorgestrel) | ella (Ulipristal Acetate) | Copper IUD (ParaGard) |
|---|---|---|---|
| Optimal Window | Up to 72 hours [1.4.5] | Up to 120 hours [1.5.2] | Up to 120 hours [1.5.2] |
| Effectiveness | 75-89% [1.5.1] | More effective than Plan B, especially closer to ovulation [1.5.1, 1.5.4] | Over 99% effective [1.5.1, 1.7.3] |
| BMI Impact | May be less effective for individuals over 165 lbs [1.6.1] | More effective than Plan B for higher BMI, but may be less effective over 195 lbs [1.6.1] | Not affected by BMI [1.7.2] |
| Access | Over-the-counter [1.6.1] | Prescription only [1.5.4] | Requires provider insertion [1.5.4] |
| Key Interactions | St. John's Wort, certain seizure meds, rifampin [1.2.2] | Similar interactions as Plan B; hormonal birth control should be paused [1.9.3] | None of this type [1.5.1] |
Conclusion
While Plan B is a safe and effective option for emergency contraception for many, its success depends on proper use and an absence of interacting substances. The most significant items to avoid are the herbal supplement St. John's Wort and specific prescription medications like certain anticonvulsants, the antibiotic rifampin, and some HIV drugs. Always disclose all medications and supplements you are taking to your pharmacist or doctor to ensure you are choosing the most effective and appropriate form of emergency contraception for your situation.
For more information, consult a healthcare provider or a resource like Planned Parenthood.
