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Drugs and Lactation Database (LactMed®) [Internet]. Bethesda (MD): National Institute of Child Health and Human Development; 2006-.

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Drugs and Lactation Database (LactMed®) [Internet].

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Last Revision: March 17, 2021.

Estimated reading time: 2 minutes

CASRN: 8064-90-2

image 134990779 in the ncbi pubchem database

Drug Levels and Effects

Summary of Use during Lactation

With healthy, fullterm infants it appears acceptable to use sulfamethoxazole and trimethoprim during breastfeeding after the newborn period. The time of greatest risk for hemolysis in fullterm newborns without glucose-6-phosphate dehydrogenase (G6PD) deficiency might be as short as 8 days after birth.[1] 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. Sulfamethoxazole and trimethoprim should be avoided while breastfeeding a G6PD-deficient infant.[2,3]

Drug Levels

Maternal Levels. In 20 mothers in the immediate postpartum period given oral trimethoprim, peak milk levels occurred 3 hours after the dose. In 14 of these women who received a daily dosage of 320 mg, the peak milk level averaged 2.4 mg/L and the trough averaged 1 mg/L. In 6 other women who received a daily dosage of 480 mg, the peak milk level averaged 4 mg/L and the trough averaged 1.5 mg/L. The authors calculated that a breastfed infant would receive a daily dosage of 0.75 mg with a maternal dosage of 320 mg daily and 1.7 mg with a maternal dosage of 480 mg.[4,5]

Forty women who were 5 or fewer days postpartum period received oral co-trimoxazole equivalent to 800 mg of sulfamethoxazole and 320 mg of trimethoprim twice daily. Another 10 women were given this dose 3 times daily. Milk levels were measured several times daily for 5 days. After trimethoprim doses of 320 mg daily, average milk levels were about 2 mg/L in both groups. Milk levels increased to about 3 mg/L by day 5 of therapy. Average sulfamethoxazole milk levels were 4.5 and 5.3 mg/L, respectively, with the 2 doses.[6] With the usual dose of trimethoprim 320 mg and sulfamethoxazole 800 mg daily, an exclusively breastfed infant would be expected to receive 0.45 mg/kg daily of trimethoprim and 0.68 mg/kg daily of sulfamethoxazole. This is very low in comparison to the established treatment dosages of 8 mg/kg and 40 mg/kg daily for infants over 2 months of age.

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

Effects in Breastfed Infants

An extensive systematic review of the use of sulfonamides near term and during breastfeeding found no adverse reactions in infants. The authors concluded that use of sulfamethoxazole-trimethoprim during breastfeeding presents no risk of neonatal kernicterus.[7]

A prospective, controlled study asked mothers who called an information service about adverse reactions experience by their breastfed infants. Of 12 women who took sulfamethoxazole and trimethoprim during breastfeeding (time postpartum and dosage not stated), none reported diarrhea, drowsiness or irritability in her infant. Two mothers reported poor feeding in their infants.[8]

Effects on Lactation and Breastmilk

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


Zao J, Koren G, Bozzo P. Using nitrofurantoin while breastfeeding a newborn. Can Fam Physician. 2014;60:539–40. [PMC free article: PMC4055319] [PubMed: 24925943]
Chung AM, Reed MD, Blumer JL. Antibiotics and breast-feeding: A critical review of the literature. Paediatr Drugs. 2002;4:817–37. [PubMed: 12431134]
Farahnik B, Murase JE. Antibiotic safety considerations in methicillin-resistant Staphylococcus aureus postpartum mastitis. J Am Acad Dermatol. 2016;75:e149. [PubMed: 27646759]
Arnauld R, Soutoul JH, Gallier J, et al. Ouest Med. 1972;25:959–64. [Study on the passage of trimethoprim into mother's milk]
Borderon E, Soutoul JH, Borderon JC, et al. Med Mal Infect. 1975;5:373–6. [Excretion of antibiotics in human milk]
Miller RD, Salter AJ. The passage of trimethoprim/sulfamethoxazole into breast milk and its significance. In, Daikos CK, ed. Progress in Chemotherapy. Antibacterial chemotherapy 1974;1:687-91.
Forna F, McConnell M, Kitabire FN, et al. Systematic review of the safety of trimethoprim-sulfamethoxazole for prophylaxis in HIV-infected pregnant women: Implications for resource-limited settings. AIDS Rev. 2006;8:24–36. [PubMed: 16736949]
Ito S, Blajchman A, Stephenson M, et al. Prospective follow-up of adverse reactions in breast-fed infants exposed to maternal medication. Am J Obstet Gynecol. 1993;168:1393–9. [PubMed: 8498418]

Substance Identification

Substance Name


CAS Registry Number


Drug Class

Breast Feeding


Anti-Infective Agents

Anti-Infective Agents, Urinary

Antibacterial Agents

Folic Acid Antagonists


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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Attribution Statement: LactMed is a registered trademark of the U.S. Department of Health and Human Services.

Bookshelf ID: NBK501289PMID: 30000348


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