No, Antibiotics Do Not Cancel Out Plan B
Taking broad-spectrum antibiotics concurrently with Plan B (levonorgestrel) does NOT reduce the effectiveness of emergency contraception. 1
Evidence from Guidelines
The U.S. Medical Eligibility Criteria for Contraceptive Use explicitly classifies broad-spectrum antibiotics as Category 1 for all hormonal contraceptive methods, including emergency contraception. 1 Category 1 means there is no restriction for use—no known interaction exists between these medications. 1
This classification applies to:
The One Important Exception: Rifamycin Antibiotics
The only antibiotics that may reduce hormonal contraceptive effectiveness are rifampin (rifampicin) and rifabutin, which are classified as Category 3 for combined hormonal contraceptives. 1 However, these medications:
- Are NOT broad-spectrum antibiotics 1
- Are primarily used for tuberculosis and certain other specific infections 1
- Are rarely prescribed in routine clinical practice 1
- Even for these drugs, the evidence shows no interaction with levonorgestrel IUDs or copper IUDs (Category 1) 1
Clinical Bottom Line
Plan B will work as intended when taken with common antibiotics like amoxicillin, azithromycin, doxycycline, ciprofloxacin, or cephalosporins. 1 The patient should:
- Take Plan B as soon as possible after unprotected intercourse (ideally within 24 hours, but effective up to 120 hours) 2
- Not delay taking emergency contraception due to antibiotic concerns 2
- Use backup barrier contraception (condoms) for 7 days if starting or resuming regular hormonal contraception after Plan B 2
Common Pitfall to Avoid
This is a persistent myth in contraceptive counseling. The confusion likely stems from the well-documented interaction between rifampin and regular hormonal contraceptives, which has been incorrectly generalized to all antibiotics. 1 Unless the patient is specifically taking rifampin or rifabutin for tuberculosis or similar conditions, antibiotic use poses no threat to emergency contraception efficacy. 1