What is the pregnancy category of Flagyl (metronidazole)?

Metronidazole (Flagyl) Pregnancy Category

Metronidazole (Flagyl) is classified as FDA Pregnancy Category B, indicating no evidence of risk in humans despite showing carcinogenic activity in animal studies. 1

Understanding Pregnancy Category B Classification

Metronidazole has been extensively studied in pregnancy with the following key findings:

• The FDA classifies metronidazole as pregnancy category B, meaning reproduction studies in animals have revealed no evidence of harm to the fetus, but adequate and well-controlled studies in pregnant women are not available 1
• Metronidazole crosses the placental barrier rapidly and enters fetal circulation 1
• Animal studies have shown no evidence of impaired fertility or harm to the fetus at doses up to five times the human dose 1

Trimester-Specific Considerations

First Trimester

• The CDC recommends avoiding metronidazole during the first trimester when possible 2
• However, for trichomoniasis treatment during pregnancy, metronidazole can be used when alternative treatments have been inadequate 1

Second and Third Trimesters

• Metronidazole is considered safe and is recommended for treatment of bacterial vaginosis and other infections during the second and third trimesters 2
• The CDC recommends metronidazole 500 mg orally twice daily for 7 days for pregnant women with bacterial vaginosis 2

Pharmacokinetics in Pregnancy

• Studies show that the pharmacokinetic profile of metronidazole does not significantly change at different stages of pregnancy 3
• Weight-corrected estimates of exposure, maximum plasma drug concentration, and area under the plasma concentration curve are similar between pregnant women at early, middle, and late stages of pregnancy 3

Safety Profile

• Reproduction studies have revealed no evidence of harm to the fetus due to metronidazole 1
• No fetotoxicity was observed when metronidazole was administered orally to pregnant mice at doses approximately 1.5 times the most frequently recommended human dose 1
• A 2021 study found that metronidazole use during pregnancy was not associated with birth defects or congenital abnormalities 4

Clinical Applications During Pregnancy

• Metronidazole is recommended for treating bacterial vaginosis, trichomoniasis, and other anaerobic infections during pregnancy 5, 2
• For trichomoniasis, patients can be treated with 2 g of metronidazole in a single dose 5
• For bacterial vaginosis, the recommended regimen is metronidazole 500 mg orally twice daily for 7 days 2

Important Precautions

• Patients must avoid alcohol during metronidazole treatment and for 24 hours afterward to prevent disulfiram-like reactions 2
• Metronidazole is secreted in human milk in concentrations similar to those found in plasma, so a decision should be made whether to discontinue nursing or the drug 1
• Common side effects include mild-to-moderate gastrointestinal disturbance and unpleasant taste 2

Summary

Metronidazole is classified as FDA Pregnancy Category B, making it generally acceptable for use during pregnancy when clinically indicated, with particular caution recommended during the first trimester. The benefit-risk assessment should be considered, especially for infections where metronidazole is the treatment of choice.