Cephalexin

Pregnancy

MotherToBaby reports limited pregnancy evidence and mixed miscarriage findings. A Norwegian population-based cohort (Boissiere-O'Neill et al., online 2025, journal issue 2026) included 704,082 pregnancies from 2009 to 2018; 91,836 had systemic antimicrobial exposure in early pregnancy and 824 were exposed to cephalexin. In the cephalexin group, 138 miscarriages were reported, with an adjusted HR of 1.87 (95% CI 1.57-2.22). The authors' bias analysis found the association approached the null only in scenarios where the underlying infection carried high miscarriage risk, and they called for further study. A Quebec population-based cohort (Muanda et al., 2017) included 139,938 live births; 13 major congenital malformations occurred among 124 cephalexin-exposed pregnancies (10.48%), with adjusted OR 0.83 (95% CI 0.47-1.46). A Hungarian case-control study (Czeizel et al., 2001) included 22,865 cases, 38,151 population controls, and 812 Down syndrome controls. Cephalosporin use was mainly oral cephalexin, and the study found no detectable teratogenic risk. The current KEFLEX label says published epidemiologic studies and pharmacovigilance reports over several decades have not established drug-associated risks, but notes small samples, retrospective data, and inconsistent comparator groups. MotherToBaby reports no cephalexin-specific studies of early delivery, low birth weight, or later behavior or learning.

Breastfeeding

LactMed reports low milk levels and generally no expected infant effects, while noting occasional diarrhea or thrush and a rare severe skin reaction after prior cefazolin exposure. In Kafetzis et al. (1981), 42 lactating mothers received one of several antibiotics; the abstract reports a mean cephalexin peak milk level of 0.50 +/- 0.23 microgram/mL at 4 hours but does not state the number of cephalexin recipients. Ilett et al. (2006) reported one mother-infant case in which the mother took cephalexin with probenecid; the estimated relative infant dose for cephalexin was 0.5% (112 micrograms/kg/day), and the infant had severe diarrhea. The co-medication prevents assigning the effect to cephalexin alone. LactMed reports one diarrhea case in an 11-infant cephalexin control group in Benyamini et al. (2005); the PubMed abstract reports a 9% overall adverse-effect rate for controls but does not name the symptom. The Ito et al. (1993) abstract describes a broader cohort of 838 breastfed infants and reports minor effects with antibiotics as a class, without giving a cephalexin-specific subgroup. Chu et al. (2019) described one 4-month-old infant previously treated with intravenous cefazolin; the mother had taken cephalexin for 2 days before the infant's skin reaction. The authors called cephalexin through milk a possible cause, found a positive lymphocyte transformation test for both drugs, and said they could not rule out a false-positive result. LactMed reported no published infant serum levels or milk-production data as of its November 2024 revision.

What was tested

Pregnancy evidence includes observational studies of first-trimester antibiotic exposure and birth defects, plus a Norwegian registry cohort examining miscarriage after early-pregnancy exposure. These studies do not provide separate outcome estimates for the second or third trimester, early delivery, or low birth weight. Breastfeeding evidence includes maternal milk concentration measurements, small reports of infant symptoms, and one rare severe case report; it does not establish how often such effects occur. A 2026 systematic review of three randomized trials involving 1,144 patients studied cephalexin plus metronidazole after cesarean delivery to prevent maternal infection, not fetal outcomes or infant effects through milk.