Is Flagyl (metronidazole) safe to use during pregnancy?

From the Guidelines

Metronidazole (Flagyl) is generally considered safe during pregnancy when medically necessary, though it's typically recommended to avoid it during the first trimester if possible. The medication is classified as FDA Pregnancy Category B, meaning animal studies have not shown risk to the fetus, but there aren't adequate well-controlled studies in pregnant women. For bacterial vaginosis or trichomoniasis during pregnancy, typical dosing is 500 mg orally twice daily for 7 days, or 250 mg three times daily for 7 days.

Key Considerations

• For more severe infections, treatment should be individualized by a healthcare provider.
• While some older studies raised theoretical concerns, multiple large studies and meta-analyses have not demonstrated increased risks of birth defects, preterm birth, or other adverse pregnancy outcomes with metronidazole use 1.
• The benefits of treating certain infections during pregnancy often outweigh potential risks, as untreated infections can lead to complications like preterm labor or premature rupture of membranes.
• Always consult with your healthcare provider before taking any medication during pregnancy, as they can evaluate your specific situation and determine the appropriate treatment approach.

Important Details

• Metronidazole can be used during pregnancy and lactation if there are no other safer alternatives 1.
• Present in breast milk: if a single 2 g oral dose is used, stop feeding for 12–24 h after dose.
• Associated with fetal damage in animals, but no reports confirmed in humans 1.
• Long-term therapy of the mother could result in risk of neonatal bleeding by inhibition of vitamin K synthesis: treat mother and neonate with phytomenadione (vitamin K).

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

Flagyl (metronidazole) should be used during pregnancy only if clearly needed. The drug crosses the placental barrier and its effects on human fetal organogenesis are not known. There are no adequate and well-controlled studies in pregnant women, and metronidazole is a carcinogen in rodents. Use for trichomoniasis during pregnancy should be restricted to those in whom alternative treatment has been inadequate, and use in the first trimester should be carefully evaluated 2.

From the Research

Safety of Flagyl During Pregnancy

• The safety of metronidazole, also known as Flagyl, during pregnancy has been studied in various research papers 3, 4.
• According to a literature review, metronidazole was found to be effective in preventing preterm births when used in conjunction with other antibiotics, and its use did not predict birth defects or congenital abnormalities 3.
• However, the use of metronidazole was associated with a 70% increased risk of spontaneous abortion, although this risk should be interpreted cautiously due to the severity of genitourinary infection 3.
• Another study reviewed the literature on the carcinogenic and teratogenic potential of metronidazole and discussed the evidence that associates trichomoniasis and bacterial vaginosis with perinatal outcomes, providing current practice recommendations 4.

Use of Metronidazole in Pregnancy

• Metronidazole is the recommended treatment for trichomoniasis and bacterial vaginosis in nonpregnant women, but its use in pregnancy has been a topic of concern due to potential adverse effects on the fetus 4.
• The CDC's guidelines advocate for the use of metronidazole for the treatment of bacterial vaginosis in pregnant women 3.
• A study published in 2001 reviewed the current knowledge of the epidemiology of drug use among pregnant women, drug metabolism in pregnancy, adverse fetal and neonatal effects of drugs, and specific effects of drugs that are relatively or absolutely contraindicated in pregnancy, including metronidazole 5.

Treatment of Trichomoniasis in Women

• Trichomoniasis is the most common non-viral sexually transmitted infection worldwide, and is associated with serious reproductive morbidity, poor birth outcomes, and amplified HIV transmission 6.
• Single-dose metronidazole is the first-line treatment for trichomoniasis, but a 7-day dose may be more effective in clearing the infection, especially in women with bacterial vaginosis 6.
• The use of metronidazole in pregnancy should be carefully considered, taking into account the potential benefits and risks, as well as the severity of the genitourinary infection 3, 4.