Is metronidazole (Flagyl) safe during pregnancy?

From the Guidelines

Flagyl (metronidazole) is generally considered safe during pregnancy when medically necessary, though it's typically avoided in the first trimester if possible. For bacterial vaginosis or trichomoniasis infections during pregnancy, oral metronidazole 500 mg twice daily for 7 days is the standard regimen, as recommended by the Centers for Disease Control and Prevention 1. This regimen is supported by evidence from multiple studies, including a guideline published in the MMWR Recommendations and Reports 1, which suggests that metronidazole can be used to treat bacterial vaginosis in pregnant women, particularly those at high risk for preterm delivery.

Key Considerations

• The medication works by disrupting bacterial DNA, effectively treating certain infections that could otherwise pose greater risks to pregnancy if left untreated.
• Common side effects include metallic taste, nausea, and gastrointestinal discomfort.
• Patients should avoid alcohol completely while taking metronidazole and for 48 hours afterward due to potential severe reactions.
• Always consult with your healthcare provider before taking any medication during pregnancy, as they can weigh the specific benefits against potential risks for your individual situation.

Evidence Summary

The evidence from studies published in reputable journals such as the Annals of Internal Medicine 1 and American Family Physician 1 supports the use of metronidazole in pregnancy for the treatment of bacterial vaginosis and trichomoniasis. However, it's essential to follow the recommended regimens and consult with a healthcare provider to minimize potential risks. The most recent and highest quality study 1 provides the basis for the recommended treatment regimen, emphasizing the importance of evidence-based practice in clinical decision-making.

From the FDA Drug Label

Pregnancy Teratogenic Effects Pregnancy category B Metronidazole crosses the placental barrier and enters the fetal circulation rapidly. Reproduction studies have been performed in rats at doses up to five times the human dose and have revealed no evidence of impaired fertility or harm to the fetus due to metronidazole No fetotoxicity was observed when metronidazole was administered orally to pregnant mice at 20 mg/kg/day approximately one and a half times the most frequently recommended human dose (750 mg/day) based on mg/kg body weight; however in a single small study where the drug was administered intraperitoneally, some intrauterine deaths were observed The relationship of these findings to the drug is unknown. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, and because metronidazole is a carcinogen in rodents, this drug should be used during pregnancy only if clearly needed Use of metronidazole for trichomoniasis during pregnancy should be restricted to those in whom alternative treatment has been inadequate. Use of metronidazole for trichomoniasis in the first trimester of pregnancy should be carefully evaluated because metronidazole crosses the placental barrier and its effects on the human fetal organogenesis are not known

Metronidazole is classified as a Pregnancy Category B drug.

• It crosses the placental barrier and enters the fetal circulation rapidly.
• There are no adequate and well-controlled studies in pregnant women.
• Use during pregnancy should be restricted to cases where alternative treatment has been inadequate.
• The decision to use metronidazole during pregnancy should be made with caution, considering the potential risks and benefits 2.

From the Research

Safety of Flagyl in Pregnancy

• Flagyl, also known as metronidazole, is used to treat various infections, including bacterial vaginosis and trichomoniasis, in pregnant women 3, 4, 5.
• According to a study published in 2015, treatment of symptomatic Trichomonas vaginalis with oral metronidazole is warranted in pregnancy for the prevention of preterm birth 3.
• A 2021 literature review found that metronidazole is effective in preventing preterm births when used in conjunction with other antibiotics, and its use does not predict birth defects or congenital abnormalities 5.
• However, the same review found that metronidazole is associated with a 70% increased risk of spontaneous abortion, although this risk should be interpreted cautiously due to the severity of genitourinary infection 5.
• Other studies have compared the efficacy and safety of metronidazole with other treatments, such as clindamycin, for bacterial vaginosis, and found that metronidazole is a safe and effective option 6, 7.

Treatment Guidelines

• The 2015 study recommends treating symptomatic bacterial vaginosis with oral metronidazole 500 mg twice daily for 7 days, with alternatives including vaginal metronidazole gel and oral or vaginal clindamycin cream 3.
• The study also recommends that partner treatment, even without screening, enhances cure rates for trichomoniasis 3.
• The 2021 review supports the use of metronidazole in pregnancy, as advocated by CDC's guidelines, for the treatment of bacterial vaginosis 5.

Adverse Effects

• Adverse reactions to metronidazole are infrequent and mild, with some patients experiencing non-bloody diarrhea, nausea, and taste perversion 6, 7.
• The 2021 review found that metronidazole is associated with a 70% increased risk of spontaneous abortion, although this risk should be interpreted cautiously due to the severity of genitourinary infection 5.