Cephalexin is excreted in breast milk.
The relative infant dose (RID) of cephalexin is 0.13% to 0.52% when compared to an infant therapeutic dose of 25 to 100 mg/kg/day. In general, breastfeeding is considered acceptable when the relative infant dose is <10%; when an RID is >25% breastfeeding should generally be avoided (Anderson 2016; Ito 2000). Using a milk concentration of 0.85 mcg/mL, the estimated daily infant dose via breast milk is 0.13 mg/kg/day. This milk concentration was obtained following a single maternal dose of cephalexin 1,000 mg orally on the third postpartum day (Kafetzis 1981).
The mean peak milk concentration occurred 4 to 5 hours after the dose (Kafetzis 1981). Slightly higher concentrations of cephalexin were detected in the breast milk of a lactating woman also administered probenecid and cephalexin for ≥16 days (Ilett 2006).
Diarrhea has been reported in breastfeeding infants (Ilett 2006; Ito 1993). In general, antibiotics that are present in breast milk may cause nondose-related modification of bowel flora. Monitor infants for GI disturbances (WHO 2002).
When an antibiotic is needed, cephalexin may be used to treat mastitis in breastfeeding women allergic to preferred agents (Amir 2014; Berens 2015). The manufacturer recommends that caution be exercised when administering cephalexin to nursing women.