This drug should be used during pregnancy only if the benefit outweighs the risk to the fetus.
-According to some authorities: Use is contraindicated.

AU TGA pregnancy category: B3
US FDA pregnancy category: Not assigned.

Risk summary: Insufficient data available on use of this drug in pregnant women to inform a drug-related risk.

Comment:
-This drug may interfere with folic acid metabolism; folic acid supplementation may be needed.

Animal studies have revealed evidence of teratogenicity and fetal loss at high doses; birth defects typical of folic acid antagonism have been observed in animal studies when very high doses were given during organ development. Teratogenicity has been observed in rats administered doses 40 times the human dose; in rabbits, the overall increase in fetal loss (dead and resorbed and malformed conceptuses) was associated with doses 6 times the human therapeutic dose. This drug crosses the placental barrier. There are no controlled data in human pregnancy.

In 1 retrospective study of mothers given either placebo or oral sulfamethoxazole-trimethoprim, the outcome of 186 pregnancies showed congenital abnormalities in 3 of 66 patients using placebo and in 4 of 120 patients using sulfamethoxazole-trimethoprim; no abnormalities reported in 10 children whose mothers used the drug during the first trimester. In a separate survey, the same authors found no congenital abnormalities in 35 children whose mothers used oral sulfamethoxazole-trimethoprim at time of conception or shortly thereafter.

AU TGA pregnancy category B3: Drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals have shown evidence of an increased occurrence of fetal damage, the significance of which is considered uncertain in humans.

US FDA pregnancy category Not Assigned: The US FDA has amended the pregnancy labeling rule for prescription drug products to require labeling that includes a summary of risk, a discussion of the data supporting that summary, and relevant information to help health care providers make prescribing decisions and counsel women about the use of drugs during pregnancy. Pregnancy categories A, B, C, D, and X are being phased out.

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In the immediate postpartum period, 20 mothers were given this drug orally; peak milk levels occurred 3 hours after dosing. In 14 of these women who received 320 mg/day, peak and trough milk levels averaged 2.4 and 1 mg/L, respectively; in 6 other women who received 480 mg/day, peak and trough milk levels averaged 4 and 1.5 mg/L, respectively. According to author calculation, a breastfed infant would receive 0.75 mg/day with a maternal dose of 320 mg/day and 1.7 mg/day with a maternal dose of 480 mg/day.

In the early postpartum period, 40 women received oral sulfamethoxazole-trimethoprim and had milk levels measured several times per day for 5 days. After trimethoprim doses of 320 mg/day, milk levels averaged about 2 mg/L; milk levels increased to about 3 mg/L by day 5 of therapy. In 10 other women who received trimethoprim doses of 480 mg/day, average milk levels were only slightly higher. With the usual dose of 320 mg/day, an exclusively breastfed infant would be expected to receive 0.45 mg/kg/day of trimethoprim; this is very low compared to established treatment doses of 8 mg/kg/day for infants older than 2 months.

In 1 study, no adverse effects were noted in infants during 4 days of maternal therapy with sulfamethoxazole-trimethoprim.

In a telephone follow-up study, 12 nursing mothers reported taking sulfamethoxazole-trimethoprim (dose not provided); 2 mothers reported poor feeding in their infants. Diarrhea was not reported among the exposed infants.

Use is considered acceptable; caution is recommended.
-According to some authorities: Breastfeeding is not recommended during use of this drug.
-According to some authorities: Use is not necessarily contraindicated for short-term therapy; according to at least 1 manufacturer, use is contraindicated if the nursing infant is younger than 4 months.

Excreted into human milk: Yes

Comments:
-Due to the low levels of this drug in breast milk, amounts ingested by the infant are small and would not be expected to cause any harmful effects in the nursing infant.
-This drug may interfere with folic acid metabolism; caution is recommended when administering to a nursing woman.
-According to at least 1 manufacturer, effects on the nursing infant are likely if therapeutic doses are administered to nursing mothers.

See references