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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: March 17, 2021.

Estimated reading time: 2 minutes

CASRN: 738-70-5

image 135269979 in the ncbi pubchem database

Drug Levels and Effects

Summary of Use during Lactation

Because of the low levels of trimethoprim in breastmilk, amounts ingested by the infant are small and would not be expected to cause any adverse effects in breastfed infants.

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.[1,2]

Forty women in the early postpartum period received oral co-trimoxazole and had milk levels measured several times daily for 5 days. After trimethoprim doses of 320 mg daily, average milk levels were about 2 mg/L. Milk levels increased to about 3 mg/L by day 5 of therapy. In 10 other women who received trimethoprim 480 mg daily, average milk levels were only slightly higher.[3] With the usual dose of trimethoprim 320 mg daily, an exclusively breastfed infant would be expected to receive 0.45 mg/kg daily of trimethoprim. This is very low in comparison to the established treatment dosages of 8 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

In one study, no adverse effects were noted in infants during 4 days of maternal therapy with co-trimoxazole.[1]

In a telephone follow-up study, 12 nursing mothers reported taking co-trimoxazole (dosage unspecified). Two mothers reported poor feeding in their infants. Diarrhea was not reported among the exposed infants.[4][4]

Effects on Lactation and Breastmilk

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

Alternate Drugs to Consider

Ciprofloxacin, Levofloxacin, Nitrofurantoin


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.
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, 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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