TL;DR
- Penicillins (amoxicillin, ampicillin, amoxicillin/clavulanate, phenoxymethylpenicillin) – compatible.
- Cephalosporins generations I–III (cephalexin, cefuroxime, ceftriaxone, cefixime) – compatible.
- Macrolides (azithromycin, clarithromycin) – compatible. Erythromycin – caution in infants under 2 weeks of age due to rare pyloric stenosis reports.
- Nitrofurantoin for UTI – compatible, except the first month of infant life and in G6PD deficiency.
- Metronidazole short courses 5–7 days – compatible. Long courses and single high doses warrant a pediatrician check.
- Tetracyclines (doxycycline) short courses up to 21 days – acceptable. Long courses avoided due to theoretical risk to dental enamel.
- Fluoroquinolones (ciprofloxacin, levofloxacin) – most data support compatibility, but a compatible alternative is preferred when clinically reasonable.
- Sulfonamides (trimethoprim/sulfamethoxazole) – compatible after the first month of infant life and outside jaundice periods.
- Chloramphenicol, tigecycline, linezolid – systemically contraindicated or used in hospital settings with temporary nursing interruption.
For a specific INN and dose, see the drug card on Evigrade and cross-check with e-lactancia.org – the free academic resource maintained by APILAM (Spanish pediatricians).
What changes during lactation
An antibiotic reaches breast milk in a concentration usually below the infant therapeutic level. The compatibility decision rests on three parameters:
- Relative infant dose (RID) – the share of maternal dose that reaches the infant through milk. Below 10% is almost always safe; most penicillins and cephalosporins yield RID < 1%.
- Oral bioavailability in the infant. Large molecules (vancomycin, parenteral aminoglycosides) are practically not absorbed when ingested orally, so even if they enter milk, no systemic effect occurs.
- Newborn safety profile. If a drug is acceptable for direct prescription to an infant, indirect exposure through milk is even safer.
Compatible classes
Penicillins
Amoxicillin, ampicillin, amoxicillin/clavulanate, phenoxymethylpenicillin, benzylpenicillin. RID 0.5–1%. Decades of safety data in infants. Rare allergic rash or loose stools in the nursing infant warrant a pediatrician call but not nursing interruption.
These are first-line for strep throat, otitis, sinusitis, skin and soft tissue infections in a nursing mother.
Cephalosporins generations I–III
Cephalexin (oral cefazolin equivalent), cefuroxime, ceftriaxone, cefixime. RID 1–2%. Compatible across all lactation stages including the newborn period. High-dose ceftriaxone theoretically competes with bilirubin for albumin binding, but at standard regimens this has no clinical impact.
Macrolides
Azithromycin and clarithromycin are first choice for suspected atypical infection (Mycoplasma, Chlamydia) and as alternatives in penicillin allergy. RID 2–4%. Compatible.
Erythromycin – isolated reports of pyloric stenosis in infants under 2 weeks of age; in older infants erythromycin is compatible, but for newborns an alternative is chosen.
Lincosamides
Clindamycin – compatible. Used for staphylococcal skin infections and MRSA mastitis. Infant stools may change; bloody stools warrant nursing pause and a clinician call.
Nitrofurantoin
First choice for uncomplicated UTI in nursing mothers beyond the first postpartum month. Contraindicated in the first month and with suspected G6PD deficiency in the infant due to hemolysis risk.
Metronidazole
Short courses 5–7 days (bacterial vaginosis, amebiasis, giardiasis) are compatible. A single 2 g dose may color the milk yellow and alter its taste; some pediatricians suggest pausing nursing for 12 hours after this dose, although no clinical harm has been documented.
For systemic anaerobic infections requiring prolonged metronidazole therapy, the approach is discussed with the pediatrician.
Classes with conditions
Tetracyclines
Doxycycline in courses up to 21 days (Lyme borreliosis, acne) is acceptable: RID 6%, milk calcium binds most of the antibiotic, infant bioavailability is minimal. Long courses are not recommended due to theoretical effects on dental enamel and bone growth.
Older tetracycline is rarely used today.
Fluoroquinolones
Ciprofloxacin, levofloxacin, moxifloxacin – previously considered contraindicated based on canine cartilage damage in beagle puppy experiments. Modern human data did not confirm this risk. RID 2–6%. Per AAP 2024 and e-lactancia, most fluoroquinolones are compatible. As a precaution, a compatible alternative is preferred when clinically reasonable (e.g., amoxicillin/clavulanate for UTI).
Sulfonamides
Trimethoprim/sulfamethoxazole is compatible after the first month of infant life and without jaundice. Sulfonamides compete with bilirubin for albumin binding, which in a jaundiced newborn raises kernicterus risk. Beyond the first month this risk no longer applies.
Avoid
Chloramphenicol
Systemically contraindicated. High milk concentrations, risk of aplastic anemia and gray syndrome in the newborn. Topical eye drops with chloramphenicol are compatible.
Tigecycline, intravenous vancomycin
Hospital-only use. The lactation decision is made by the treating clinician.
Linezolid
Compatible with nursing in some scenarios but, due to thrombocytopenia risk and rare neurologic adverse effects, prescribed only when alternatives are absent (e.g., MRSA infections).
Mastitis strategy
Lactational mastitis is the most common reason a nursing mother is prescribed an antibiotic. Per Academy of Breastfeeding Medicine Protocol #36 (2022, 2024 update):
- Continue breastfeeding or pumping from both breasts with priority on the affected side.
- Cold between feedings to reduce edema; warm shower before nursing to ease let-down.
- Paracetamol up to 4 g/day or ibuprofen up to 1.2 g/day – both compatible.
- Antibiotics if clinical infection lasts beyond 24 hours or worsens: cephalexin 500 mg four times daily or dicloxacillin 500 mg four times daily for 10–14 days. Penicillin-allergic alternative: clindamycin 300 mg four times daily.
- Suspected MRSA (prior MRSA episode, empirical β-lactam non-response) – trimethoprim/sulfamethoxazole after the first infant month, or clindamycin.
Systematic pumping after every feeding and warm soaks between feedings are usually unnecessary and may worsen edema – per the updated ABM position 2022.
Urinary tract infection strategy
Uncomplicated acute UTI in a nursing mother:
- First-line: nitrofurantoin 100 mg twice daily for 5 days (if the infant is older than 1 month).
- Alternative: fosfomycin 3 g single dose or cefixime 400 mg/day for 5 days.
- If intolerant: trimethoprim/sulfamethoxazole 160/800 mg twice daily for 3 days (infant older than 1 month, no jaundice, no G6PD deficiency).
- Fluoroquinolones – reserve for documented resistance.
Acute pyelonephritis strategy
Often requires hospitalization and intravenous antibiotics. Ceftriaxone 2 g IV once daily is the drug of choice and compatible with continued breastfeeding.
When the clinician insists on stopping nursing
Most modern antibiotics are compatible. If a pediatrician or OB-GYN recommends nursing interruption, ask for the specific INN and verify it on e-lactancia.org – a compatible alternative usually exists. If doubt remains, seek a second opinion from an IBCLC consultant or a pediatrician experienced with nursing mothers.
Summary
Most antibiotics prescribed to a nursing mother are compatible with continued breastfeeding. A nursing pause or switch to formula is needed in rare cases: systemic chloramphenicol, tigecycline, linezolid by strict indication, radiopharmaceutical isotope studies.
Before starting any antibiotic, check the specific INN on the Evigrade drug card or on e-lactancia.org. If the prescription raises doubt, ask the clinician to consider a compatible alternative.
Authors: Evigrade editorial team with Victoria Gankova, MD. Sources: e-lactancia.org (APILAM, 2024–2025), LactMed (NIH NLM Drugs and Lactation Database), Academy of Breastfeeding Medicine Protocol #36 (Mastitis Spectrum), AAP Committee on Drugs 2024 update, AEMPS CIMA. Full citations on the individual drug cards.