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Trimethoprim use while Breastfeeding

Drugs containing Trimethoprim: Bactrim, Bactrim DS, Septra, Polytrim, Septra DS, SMZ-TMP DS, Trimpex, Co-trimoxazole, Sulfatrim, Cotrim, Show all 22 »Bactrim Pediatric, Sulfatrim Pediatric, Primsol, Septra IV, Bactrim IV, Proloprim, Uroplus, Uroplus DS, Cotrim DS, Bethaprim Pediatric, Cotrim Pediatric, Bethaprim

Medically reviewed on April 11, 2017

Trimethoprim Levels and Effects while Breastfeeding

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]

Effects on Lactation and Breastmilk

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

Alternate Drugs to Consider

Ciprofloxacin, Levofloxacin, Nitrofurantoin


1. Arnauld R, Soutoul JH et al. [Study on the passage of trimethoprim into mother's milk]. Ouest Med. 1972;25:959-64.

2. Borderon E, Soutoul JH et al. [Excretion of antibiotics in human milk]. Med Mal Infect. 1975;5:373-6.

3. 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.

4. 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

Trimethoprim Identification

Substance Name


CAS Registry Number


Drug Class

Antiinfective Agents, Urinary

Antibacterial Agents

Folic Acid Antagonists

Administrative Information

LactMed Record Number



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Further information

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