Cloxacillin Safety During Breastfeeding
Cloxacillin is safe to use during breastfeeding for treating mastitis, with minimal transfer into breast milk (relative infant dose of only 0.03%) and no need to interrupt nursing. 1
Safety Profile
Cloxacillin transfers into breast milk at extremely low concentrations. The maximum milk concentration measured was only 67.6 ng/mL, resulting in a relative infant dose (RID) of 0.03%—far below the 10% threshold considered safe for breastfeeding 1. This minimal transfer is explained by cloxacillin's high plasma protein binding, which limits its penetration into breast milk.
Treatment Recommendations for Mastitis
Mothers with mastitis should continue breastfeeding while on antibiotic therapy. 2 Regular feeding or milk expression from the affected breast is actually an essential component of treatment, not a contraindication. Breastfeeding can continue on the affected side as long as the infant's mouth does not contact purulent drainage if an abscess develops 2.
Dosing for Mastitis
The standard regimen is cloxacillin 500 mg every 6 hours (four times daily) 1. This dosing has been studied in lactating women and confirmed safe for the nursing infant.
Dicloxacillin (a closely related antibiotic) is also recommended as a first-line agent for mastitis, with similarly minimal milk transfer 3, 4.
Clinical Considerations
Key points for prescribing:
- Do not interrupt breastfeeding - Continued nursing actually aids treatment by promoting milk drainage 2, 5
- Timing of doses - Ideally administer before the infant's longest sleep interval to minimize exposure 6
- Duration - Complete the full antibiotic course even if symptoms improve
- Monitor for complications - Approximately 10% of mastitis cases progress to breast abscess requiring drainage 2
Infant Monitoring
The only precaution is for infants with known hypersensitivity to penicillins 1. Otherwise, adverse effects in breastfed infants are extremely rare given the subtherapeutic concentrations transferred through milk 7.
Potential infant effects to watch for (though uncommon):
- Diarrhea or changes in stool pattern
- Candida overgrowth (thrush, diaper rash)
- Allergic reactions (rash, irritability)
Why Cloxacillin is Preferred
Cloxacillin and dicloxacillin are first-line choices because most bacterial mastitis is caused by Staphylococcus aureus resistant to beta-lactamase sensitive antibiotics 3. These beta-lactamase resistant penicillins effectively target the causative organism while maintaining excellent safety during lactation 4.
The evidence strongly supports that effective maternal treatment takes priority, and cloxacillin allows this without compromising infant safety or requiring cessation of breastfeeding.