Quick answer: Some medicines and herbal products speed up the liver enzymes that break down levonorgestrel. This can lower the amount of Plan B-type medicine in the body and may reduce its effectiveness. The most important examples include certain seizure medicines, tuberculosis medicines, some HIV treatments, and St John’s wort.
Do not panic or stop an essential prescription on your own. The safest step is to tell a qualified clinician or pharmacist exactly what you take—including supplements—so they can recommend the strongest appropriate emergency option.
Why Drug Interactions Matter
Levonorgestrel is processed partly by liver enzymes. Medicines called enzyme inducers make those enzymes work faster. As a result, levonorgestrel may leave the body more quickly, reducing exposure to the intended dose.
Interaction risk may continue for a period after the inducing medicine has been stopped. This is why “I stopped it yesterday” does not necessarily remove the concern.
Medicines and Products That May Reduce Effectiveness
Examples that may interact include:
- Some anti-seizure medicines: carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, and some others
- Tuberculosis medicines: rifampicin and rifabutin
- Some HIV medicines: particularly certain older or enzyme-inducing antiretroviral regimens
- St John’s wort: an herbal product sometimes used for mood symptoms
- Other strong enzyme-inducing medicines: depending on the exact drug and dose
This list is not complete. Brand names vary by country, and combination products can hide an interacting ingredient. Show the actual medicine box, prescription, or a clear photo to the pharmacist.
Do Ordinary Antibiotics Affect Plan B?
Most common antibiotics are not liver-enzyme inducers and are not expected to make levonorgestrel emergency contraception fail. The important antibiotic exceptions are rifampicin-like drugs used for conditions such as tuberculosis.
However, an antibiotic that causes vomiting or severe diarrhea may create an absorption problem. Ask for advice if you vomited soon after taking the emergency pill.
What About Alcohol, Coffee, Pain Relievers, or Vitamins?
Alcohol does not directly cancel levonorgestrel, but heavy drinking can lead to vomiting, missed timing, or another unprotected encounter. Ordinary caffeine, paracetamol, and routine vitamins are not known to make Plan B ineffective. Always check unusual supplements and prescription drugs individually.
What Should You Do If You Use an Interacting Medicine?
- Do not stop the regular medicine without the prescriber’s advice.
- Contact a clinician or pharmacist immediately after unprotected sex.
- Provide the medicine name, dose, reason for use, and date of the last dose.
- Ask about a copper IUD, which is not affected by enzyme-inducing medicines and is the most effective emergency contraceptive.
- If an oral method is being considered, use only the dose and product recommended for your situation.
Should You Simply Take Two Plan B Pills?
Some clinical guidelines use an adjusted levonorgestrel dose for people taking liver enzyme inducers, but the evidence is limited and this should not be improvised. A higher dose is not a universal solution for every interaction, body weight concern, or late dose.
Can You Use Mifestad Instead?
Mifestad 10 is offered as a low-dose mifepristone emergency contraceptive with a 120-hour window.
For someone not taking an interacting medicine and still within the usual window, Victoria OneStep remains a practical levonorgestrel 1.5 mg option when taken promptly.
Other Reasons a Plan B-Type Pill May Be Less Effective
- It was taken after ovulation had already occurred.
- Too much time passed after unprotected sex.
- Vomiting occurred before absorption.
- The product was expired, counterfeit, or stored incorrectly.
- More unprotected sex happened after the dose.
Testing and Follow-Up
Take a pregnancy test about three weeks after the incident. Use condoms for any sex after the emergency dose.
Frequently Asked Questions
Does amoxicillin make Plan B fail?
Amoxicillin is not a liver-enzyme inducer and is not generally expected to reduce levonorgestrel effectiveness. Vomiting or severe diarrhea can still affect absorption.
Does antidepressant medicine affect Plan B?
Most common antidepressants are not major enzyme inducers, but St John’s wort can interact. Verify the exact prescription rather than relying on the medicine category.
Can I trust a drug-interaction website?
It can be a starting point, but emergency contraception is time-sensitive and databases may not cover local brands. Confirm with a pharmacist or clinician.
Bottom line: The interaction problem is not “all medicines.” It is mainly specific enzyme-inducing drugs and absorption issues. Bring the exact medication details and get help quickly.
Medical references: FSRH—Emergency Contraception Guideline; CDC—Emergency Contraception.