Keflex (Cephalexin) and Hormonal Birth Control Interactions
Keflex (cephalexin) does not significantly reduce the effectiveness of hormonal birth control and no special precautions are needed when using these medications together.
Evidence on Antibiotics and Hormonal Contraceptives
The 2024 U.S. Selected Practice Recommendations for Contraceptive Use does not list cephalexin or non-enzyme-inducing antibiotics as medications that interact with hormonal contraceptives 1. This is the most recent and authoritative guideline on contraceptive management.
The CDC's Medical Eligibility Criteria specifically addresses antibiotic therapy and hormonal contraceptives, classifying broad-spectrum antibiotics (which includes cephalexin) as Category 1 for all progestin-only contraceptives, meaning there are no restrictions for use 1.
Key Points About Antibiotic-Contraceptive Interactions
- Only enzyme-inducing antibiotics (such as rifampin and rifabutin) are known to reduce hormonal contraceptive effectiveness
- Non-enzyme-inducing antibiotics (including cephalexin) do not have evidence-based interactions with hormonal contraceptives
- The CDC guidelines specifically categorize broad-spectrum antibiotics as safe to use with hormonal contraceptives
Research Evidence and Clinical Context
There is conflicting research on this topic:
A 2021 database review of adverse drug reactions found that unintended pregnancies were reported more commonly with antibiotics compared to control medicines, suggesting a potential signal for reduced contraceptive efficacy 2.
However, multiple clinical studies have failed to demonstrate an interaction between non-rifampin antibiotics and oral contraceptives 3. A 2002 review concluded that "all clinical trials studying the effects of concomitant antibiotic therapy (with the exception of rifampin and rifabutin) have failed to demonstrate an interaction" 3.
Important Considerations
Medications That DO Reduce Contraceptive Effectiveness
The following medications are known to potentially reduce hormonal contraceptive effectiveness and require additional precautions:
- Rifampin/rifabutin
- Certain anticonvulsants (phenytoin, carbamazepine, barbiturates, primidone, topiramate, oxcarbazepine) 1
Common Misconceptions
The belief that all antibiotics reduce contraceptive effectiveness likely stems from:
- Confusion with the well-documented interaction with rifampin-like drugs
- Case reports of contraceptive failures during antibiotic use that may represent the normal 3-9% failure rate of hormonal contraceptives
- The theoretical concern that antibiotics might disrupt gut flora involved in enterohepatic circulation of estrogens
Clinical Recommendation
Based on the most current evidence:
- No additional contraceptive precautions are needed when taking Keflex (cephalexin) with hormonal contraceptives
- Patients should continue taking their hormonal contraception as prescribed
- The normal failure rate of hormonal contraceptives (3-9% for combined hormonal methods with typical use) should be acknowledged 4
Caveat
If a patient experiences diarrhea or vomiting as a side effect of antibiotic therapy, this could potentially affect contraceptive absorption. In these specific cases, additional precautions might be warranted until symptoms resolve.