Can I take trimethoprim while pregnant or breastfeeding?
A plain-words look at what the NHS, the US drug label and LactMed say about trimethoprim.
In short
Pregnancy: You can take trimethoprim while pregnant, but it's not usually recommended during the first 12 weeks, as it may lower your folic acid levels.
Breastfeeding: You may be able to take trimethoprim during breastfeeding, but ask a pharmacist or doctor first.
Your doctor, midwife or pharmacist can help you decide.
What this means for you
Pregnancy
- Folic acid matters in the early stages of your baby's development.
- In some cases, trimethoprim is the only antibiotic suitable for an infection.
- When that happens, the benefit of taking it is likely to be greater than the risk of harm.
- Whether it's right for you is something your doctor will help you decide.
- If you need trimethoprim while trying for a baby or during the first 12 weeks of pregnancy, a doctor will usually give you a prescription for higher-dose folic acid.
Breastfeeding
- Small amounts of trimethoprim reach breast milk, and if you take it for a short time it's unlikely to give your baby side effects.
When to get help
- Side effects in breastfed babies from medicines their mother takes are rare, but talk to a healthcare professional or call 111 (NHS 111 in the UK) if anything about your baby worries you.
Talk to your doctor or pharmacist
This page is educational, not advice for your own situation. Talk to your doctor, midwife or pharmacist before you start, stop or change any medicine in pregnancy or while breastfeeding.
The details: what each source says
Trimethoprim in pregnancy
The NHS says
You can take trimethoprim while pregnant, but it's not usually recommended during the first 12 weeks, as it may lower your folic acid levels. Folic acid matters in the early stages of your baby's development.
In some cases, trimethoprim is the only antibiotic suitable for an infection. When that happens, the benefit of taking it is likely to be greater than the risk of harm. Whether it's right for you is something your doctor will help you decide.
If you need trimethoprim while trying for a baby or during the first 12 weeks of pregnancy, a doctor will usually give you a prescription for higher-dose folic acid.
In our words, from the NHS page Trimethoprim (nhs.uk). Contains public sector information licensed under the Open Government Licence v3.0 (OGL v3.0). The NHS does not endorse this site.
The US drug label says
Pregnancy Teratogenic Effects Pregnancy category C Trimethoprim has been shown to be teratogenic in the rat when given in doses 40 times the human dose. In some rabbit studies, the overall increase in fetal loss (dead and resorbed and malformed conceptuses) was associated with doses six times the human therapeutic dose. While there are no large, well-controlled studies on the use of trimethoprim in pregnant women, Brumfitt and Pursell, 3 in a retrospective study, reported the outcome of 186 pregnancies during which the mother received either placebo or trimethoprim in combination with sulfamethoxazole. The incidence of congenital abnormalities was 4.5% (3 of 66) in those who received placebo and 3.3% (4 of 120) in those receiving trimethoprim and sulfamethoxazole. There were no abnormalities in the 10 children whose mothers received the drug during the first trimester. In a separate survey, Brumfitt and Pursell also found no congenital abnormalities in 35 children whose mothers had received trimethoprim and sulfamethoxazole at the time of conception or shortly thereafter.
Prescribing label, section “Pregnancy” (opening excerpt): Trimethoprim (oral), Lupin Pharmaceuticals,Inc. Read the full section on DailyMed. Quoted verbatim; US labels are written for US prescribers and patients.
Trimethoprim while breastfeeding
The NHS says
You may be able to take trimethoprim during breastfeeding, but ask a pharmacist or doctor first.
Small amounts of trimethoprim reach breast milk, and if you take it for a short time it's unlikely to give your baby side effects.
Side effects in breastfed babies from medicines their mother takes are rare, but talk to a healthcare professional or call 111 (NHS 111 in the UK) if anything about your baby worries you.
In our words, from the NHS page Trimethoprim (nhs.uk). Contains public sector information licensed under the Open Government Licence v3.0 (OGL v3.0). The NHS does not endorse this site.
LactMed says
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.
“Summary of Use during Lactation”, LactMed record “Trimethoprim” (LM414), quoted verbatim. Everything LactMed reports about Trimethoprim while breastfeeding.
The US prescribing label says
Nursing Mothers Trimethoprim is excreted in human milk. Because trimethoprim may interfere with folic acid metabolism, caution should be exercised when trimethoprim is administered to a nursing woman.
Prescribing label, section “Nursing Mothers”: Trimethoprim (oral), Lupin Pharmaceuticals,Inc. Label on DailyMed. Quoted verbatim; US labels are written for US prescribers and patients.
Sources
- NHS, Trimethoprim (nhs.uk), page last reviewed 26 November 2025. Paraphrased. Contains public sector information licensed under the Open Government Licence v3.0.
- US drug label: Trimethoprim (oral), Lupin Pharmaceuticals,Inc., human prescription drug, label effective 4 June 2024, set ID 249e938d-f181-47c4-a082-372d65462f33. DailyMed (National Library of Medicine). Public domain; quoted verbatim.
- Drugs and Lactation Database (LactMed), National Institute of Child Health and Human Development (NICHD), via NCBI Bookshelf, record “Trimethoprim” (LM414), bulk release of 22 September 2026. LactMed on NCBI Bookshelf. Public domain; quoted verbatim. LactMed is a registered trademark of the U.S. Department of Health and Human Services.
Sources checked 26 September 2026. How we source and check every page.
Educational summary of named sources, not medical advice.