LACTOLOGY FOUNDATION

Intestinal sulfonamides (ATC A07AB) are locally acting intestinal antiinfective agents used mainly for preoperative prophylaxis and treatment of gastrointestinal infections, with activity confined largely to the gut lumen. Lactation safety is complex because sulfonamides can displace bilirubin and may theoretically increase the risk of kernicterus in vulnerable neonates, so use during breastfeeding requires careful individual risk–benefit assessment, particularly in newborn or jaundiced infants.atcddd.fhi+4

Pharmacological class and indications

A07AB comprises sulfonamide preparations formulated as intestinal antiinfectives rather than systemic antibiotics, typically used for prophylaxis of intestinal infections (for example, preoperative bowel decontamination) or for acute infectious diarrhea. Representatives include older, poorly absorbed intestinal sulfonamides such as phthalylsulfathiazole, which act locally in the gut and are not widely used in modern clinical practice in some countries.atcddd.fhi+4

Mechanism of action

Sulfonamides are structural analogues of para‑aminobenzoic acid (PABA) that competitively inhibit dihydropteroate synthase, blocking folate synthesis in susceptible bacteria and exerting a primarily bacteriostatic effect. Their activity covers various gram‑positive and gram‑negative organisms, but widespread resistance and the availability of safer systemic antibiotics have limited their use largely to specific indications or combination regimens.onlinelibrary.wiley+1

Pharmacokinetics relevant to lactation

Intestinal sulfonamides in the A07AB group are designed for local action, and some agents (such as phthalylsulfathiazole) have low systemic bioavailability, resulting in relatively low maternal plasma levels at usual doses. Systemic sulfonamides in general are well absorbed and widely distributed, including into breast milk, so any intestinal product that yields appreciable systemic exposure would provide a route for transfer into human milk.atcddd.fhi+4

Maternal toxicity and adverse effects

Sulfonamides can cause hypersensitivity reactions (rash, urticaria, Stevens–Johnson syndrome), hematologic effects (e.g. agranulocytosis), and, rarely, hepatic toxicity. When absorbed systemically, they may also interact with folate metabolism and, in combination products (such as sulfamethoxazole‑trimethoprim, although not classified in A07AB), contribute to folate deficiency or other systemic adverse effects; however, these combinations are typically classified elsewhere in the ATC system.atcddd.fhi+2

Excretion into breast milk

Sulfonamides as a class are excreted into human milk, and measurable concentrations of agents such as sulfamethoxazole have been documented. For locally acting intestinal sulfonamides, the extent of milk excretion depends on systemic absorption; where absorption is low, milk levels are likely lower, but specific quantitative data for A07AB agents are lacking.atcddd.fhi+3

Reported effects in breastfed infants

A systematic review of sulfonamide use near term and during breastfeeding found no documented cases of neonatal kernicterus attributable to sulfamethoxazole–trimethoprim exposure via breast milk in healthy full‑term infants beyond the immediate newborn period. Nonetheless, sulfonamides can displace bilirubin from plasma albumin and theoretically increase the risk of jaundice and kernicterus in neonates—especially premature, jaundiced, or ill infants—so caution is advised.drugs+2

Impact on lactation and breast milk

There is no evidence that sulfonamides directly reduce milk supply or alter the endocrine regulation of lactation. However, maternal sulfonamide therapy can influence the breast milk and infant gut microbiota and the infant antibiotic resistome, and these broader microbiome effects are a general consideration with antibiotic exposure during breastfeeding.nature+3

Clinical use in breastfeeding

Many expert sources consider short‑term sulfonamide therapy (for example, sulfamethoxazole–trimethoprim) acceptable in healthy, full‑term infants after the newborn period, but advise against use in jaundiced, ill, stressed, or premature infants and in those with glucose‑6‑phosphate dehydrogenase (G6PD) deficiency. For intestinal A07AB sulfonamides, similar principles apply: they should be used during breastfeeding only when clearly indicated and when safer alternatives (such as penicillins or cephalosporins) are unsuitable, with particular caution in the early neonatal period.atcddd.fhi+3

Infant monitoring recommendations

If an A07AB sulfonamide is used during breastfeeding, infants—especially those in the first month of life—should be monitored for jaundice, poor feeding, lethargy, and signs of hemolysis (e.g. pallor, dark urine), and treatment should be stopped and medical advice sought if such symptoms appear. For older, healthy, full‑term infants, practical monitoring focuses on gastrointestinal disturbances (diarrhea, vomiting) and oral or diaper candidiasis, as for other antibiotics used in breastfeeding.drugs+3

Risk–benefit assessment

Because of the theoretical risk of bilirubin displacement and kernicterus, sulfonamides are generally avoided near term and during early neonatal breastfeeding when alternatives exist, especially in high‑risk infants. Where an intestinal sulfonamide is clearly indicated and more suitable options are unavailable, the decision to treat should weigh the severity of maternal gastrointestinal disease and the benefits of maintaining breastfeeding against the infant’s gestational age, health status, and potential vulnerability to bilirubin or hemolytic complications.nature+4

Key counselling points

Healthcare professionals should explain that A07AB intestinal sulfonamides act locally in the gut but belong to a class that can cross into breast milk and potentially affect bilirubin handling in young infants, so their use in breastfeeding is not first‑line. Shared decision‑making should emphasize the infant’s age and health, the availability of safer alternatives, and the substantial benefits of breastfeeding, with a preference for using the lowest effective dose for the shortest duration when sulfonamides are deemed necessary.atcddd.fhi+4