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Drugs and Lactation Database (LactMed) [Internet]. Bethesda (MD): National Library of Medicine (US); 2006-.

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Drugs and Lactation Database (LactMed) [Internet].

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Last Revision: October 31, 2018.

Estimated reading time: 1 minute

CASRN: 127-69-5

Chemical structure

Drug Levels and Effects

Summary of Use during Lactation

With healthy, fullterm infants it appears acceptable to use sulfisoxazole during breastfeeding after the newborn period.[1][2] Until further data are accumulated, alternate agents should probably be used in jaundiced, ill, stressed or premature infants, because of the risk of bilirubin displacement and kernicterus. Sulfisoxazole should be avoided while breastfeeding a glucose-6-phosphate dehydrogenase (G6PD) deficient infant.

Drug Levels

Maternal Levels. Following oral sulfisoxazole doses of 1 g every 6 hours for 4 doses to 6 women (time postpartum not stated), an average total recovery of sulfisoxazole and N4-acetylsulfisoxazole of 18 mg was obtained during the 48 hours from the time of the first dose. About 77% of the drug in milk was recovered as the metabolite. This averaged 0.45% of the maternal dose (not weight-adjusted).[3] This study may have underestimated the amount excreted by a small fraction, because steady-state metabolite serum levels were not achieved during the study period.[1][4]

Infant Levels. A 9-month-old infant was breastfed twice daily during maternal treatment for a urinary tract infection (dosage not stated). Urine was collected during a 24-hour time period from the infant. The infant excreted 1.1 mg of sulfisoxazole plus N4-acetylsulfisoxazole over the time period.[3]

Effects in Breastfed Infants

Relevant published information was not found as of the revision date.

Effects on Lactation and Breastmilk

Relevant published information was not found as of the revision date.


Kauffman RE. Sulfisoxazole in human milk. Reply. J Pediatr. 1981;99:172-3.
Chung AM, Reed MD, Blumer JL. Antibiotics and breast-feeding: a critical review of the literature. Paediatr Drugs. 2002;4:817-37. [PubMed: 12431134]
Kauffman RE, O'Brien C, Gilford P. Sulfisoxazole secretion into human milk. J Pediatr. 1980;97:839-41. [PubMed: 7431180]
Elliott GT, Quinn SL. Sulfisoxazole in human milk. J Pediatr. 1981;99:171-3. Letter. [PubMed: 7252664]

Substance Identification

Substance Name


CAS Registry Number


Drug Class

  • Breast Feeding
  • Anti-Infective Agents
  • Antibacterial Agents
  • Sulfonamides

Disclaimer: Information presented in this database is not meant as a substitute for professional judgment. You should consult your healthcare provider for breastfeeding advice related to your particular situation. The U.S. government does not warrant or assume any liability or responsibility for the accuracy or completeness of the information on this Site.


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