Metronidazole (Flagyl) During Pregnancy: Is It Safe?

What does the evidence say about metronidazole during pregnancy?

1. Based on nearly four decades of published research, metronidazole is not associated with birth defects regardless of which trimester it's used in. 1
2. In a study of more than 5,000 pregnant women who took metronidazole, researchers did not find an increased chance of birth defects, and the medication isn't expected to raise the chance of preterm delivery or low birth weight. 2
3. Because untreated bacterial vaginosis and trichomoniasis are themselves linked to preterm birth and low birth weight, managing those infections with metronidazole during pregnancy may lower the chance of those outcomes rather than raise it. 1
4. A smaller study of 53 pregnancies flagged a possible increased miscarriage risk, but researchers couldn't determine whether that was related to the medication itself or to the infection being addressed, and the much larger studies did not confirm it. 3
5. While nursing, infants receive doses well below a therapeutic dose given to an infant directly, though case reports describe occasional diarrhea or oral thrush; a common recommendation is to pause breastfeeding for 12 to 24 hours only after a single large 2 gram dose, not after the smaller doses used in a typical multi-day course. 4

Is metronidazole safe in each trimester?

• First trimester. Metronidazole hasn't been linked to a higher chance of birth defects when used in the first trimester, based on research spanning more than 5,000 pregnancies. A smaller, older study raised a possible miscarriage question, but researchers couldn't tell whether that was tied to the medication or the infection itself, and larger studies didn't confirm it.
• Second trimester. This is generally considered a straightforward window for metronidazole if you need it. Bacterial vaginosis and trichomoniasis, two of the infections it addresses, are themselves linked to preterm birth and low birth weight, so managing a confirmed infection may lower those risks rather than raise them.
• Third trimester. No trimester-specific restrictions are noted for metronidazole in the third trimester either. If you're managing a vaginal infection close to delivery, finishing the full course your provider prescribes matters for clearing the infection completely.