Drug Levels and Effects:

Summary of Use during Lactation:

With healthy, full-term infants it appears acceptable to use sulfamethoxazole and trimethoprim during breastfeeding after the newborn period. 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.[1]

Drug Levels:

Maternal Levels.

In 20 mothers in the immediate postpartum period given oral trimethoprim, peak milk levels occurred3hours 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.[2][3]

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 dose3times 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 about3mg/L by day 5 of therapy. Average sulfamethoxazole milk levels were 4.5 and 5.3mg/L, respectively, with the 2 doses.[4] 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-trimethopril during breastfeeding presents no risk of neonatal kernicterus.[5]

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.[6]

Possible Effects on Lactation:

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

Alternate Drugs to Consider:

Dependent on the condition being treated.


1. Chung AM, Reed MD, Blumer JL. Antibiotics and breast-feeding: a critical review of the literature. Paediatr Drugs. 2002;4:817-37. PMID:12431134
2. Arnauld R, Soutoul JH, Gallier J et al. [Study on the passage of trimethoprin into mother's milk]. Ouest Med. 1972;25:959-64.4.
3. Borderon E, Soutoul JH et al. [Excretion of antibiotics in human milk.. Med Mal Infect. 1975;5:373-6.
4. 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.
5. 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. PMID:16736949
6. 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. PMID:8498418

Substance Identification:

Substance Name:


CAS Registry Number:


Drug Class:

  • Antiinfective Agents

  • Antiinfective Agents, Urinary

  • Antibacterial Agents

  • FolicAcid
  • Antagonists

  • Sulfonamides

  • Administrative Information:

    LactMed Record Number:


    Last Revision Date:

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