Metronidazole Use During Pregnancy
Metronidazole should be avoided during the first trimester of pregnancy but can be safely used after the first trimester when clinically indicated. 1
Safety Profile by Trimester
First Trimester
- Metronidazole is contraindicated during the first trimester of pregnancy 1
- This recommendation is based on the precautionary principle, as the drug crosses the placental barrier rapidly 2
Second and Third Trimesters
- Metronidazole can be safely administered after the first trimester 1
- For trichomoniasis treatment after first trimester: 2g oral metronidazole as a single dose is recommended 1
- For bacterial vaginosis during pregnancy: metronidazole 250mg orally three times daily for 7 days is preferred 1
Evidence on Safety
- The FDA classifies metronidazole as pregnancy category B, indicating no evidence of harm to the fetus in animal studies 2
- Multiple studies and meta-analyses have not demonstrated a consistent association between metronidazole use during pregnancy and teratogenic or mutagenic effects in newborns 1
- A prospective controlled cohort study of 228 women exposed to metronidazole in pregnancy (86.2% with first-trimester exposure) found no difference in the rate of major malformations compared to controls (1.6% vs. 1.4%) 3
- A review of nearly four decades of literature indicates that metronidazole is not teratogenic, regardless of the trimester in which it is used 4
Clinical Indications During Pregnancy
- Trichomoniasis: Treatment is recommended for symptomatic pregnant women, particularly after the first trimester 1
- Bacterial vaginosis: Treatment is recommended for all symptomatic pregnant women 1
- Inflammatory bowel disease: Metronidazole can be given for pouchitis, perianal Crohn's disease, or intra-abdominal abscesses 1
Potential Benefits of Treatment
- Treatment of bacterial vaginosis in high-risk pregnant women (those with previous preterm delivery) has been shown to reduce preterm delivery in multiple randomized controlled trials 1
- Untreated bacterial vaginosis and trichomoniasis are associated with adverse pregnancy outcomes including premature rupture of membranes, preterm labor, and preterm birth 1, 4
Dosing Considerations
- For bacterial vaginosis during pregnancy: 250mg orally three times daily for 7 days (lower than the non-pregnant dose of 500mg twice daily) 1
- For trichomoniasis after first trimester: 2g orally in a single dose 1
Cautions and Monitoring
- Patients should be advised to avoid alcohol during treatment with metronidazole and for 24 hours afterward 5
- For high-risk pregnant women treated for bacterial vaginosis, a follow-up evaluation 1 month after completion of treatment should be considered 1
Important Caveats
- Some evidence suggests that metronidazole used alone in the second trimester may be associated with increased risk of preterm delivery in high-risk populations 6
- Clindamycin 300mg orally twice daily for 7 days is an alternative for patients with allergy or intolerance to metronidazole 1
The decision to use metronidazole during pregnancy should balance the risks of untreated infection against the potential risks of the medication, with particular caution during the first trimester.