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Drug Info Maternal Matters

Is it safe to take metronidazole during pregnancy?

Most research shows it’s unlikely to harm a fetus in the second and third trimester
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By | May 6, 2024 Medically reviewed by Kimberly Bowman, MD

Key takeaways

  • Some research suggests a possible link between metronidazole and birth abnormalities and pregnancy complications like preterm delivery. However, the studies are animal studies or have inconclusive evidence.

  • While metronidazole is unlikely to cause harm to fetuses, it can still cause side effects like stomach upset, diarrhea, and nausea.

  • There are alternative medications to metronidazole, such as Tindamax (tinidazole) and Cleocin (clindamycin).

In this article:

Metronidazole, sold under brand names including Flagyl and Metrosa, is an antibiotic used to treat infectious diseases of the reproductive system, gastrointestinal tract, respiratory tract, skin, and other parts of the body. It’s FDA approved to treat parasitic infections and sexually transmitted diseases such as trichomoniasis (Trichomonas vaginalis), and it’s one of the medications recommended for the treatment of bacterial vaginosis (BV)—a vaginal infection that is common during pregnancy. Most of the current available studies indicate that metronidazole is unlikely to cause harm to a fetus during pregnancy or increase the risk of pregnancy complications. Read on to learn more about the safety of metronidazole use in pregnancy, its potential risks, and alternatives for treating infections during pregnancy.

Can you take metronidazole while pregnant?

“Metronidazole is generally considered safe for use during pregnancy, particularly during the second and third trimesters; however, it’s vital to use it only when necessary and under the guidance of your doctor,” says Ila Dayananda, a board-certified OB-GYN and Chief Medical Officer at Oula maternity care.

Most available studies suggest a low risk of preterm birth, teratogenicity, congenital disabilities, or fetal malformations when taking metronidazole during pregnancy. However, it is important to note that the research is animal-based. No adequate, well-controlled studies have been conducted on pregnant women. This is why metronidazole is classified as an FDA Pregnancy Category B drug.

“The benefits of treating some infections with metronidazole usually outweigh the potential risks to the developing fetus,” Dr. Dayananda says. Infections like bacterial vaginosis are common during pregnancy, and it’s important to get the right treatment because leaving such infections untreated can sometimes “increase the risk of complications such as preterm birth, low birth weight, and potentially serious infections in the uterus or newborn,” Dr. Dayananda adds.

What are the risks of taking metronidazole while pregnant?

According to the U.S. Centers for Disease Control and Prevention (CDC), metronidazole may be prescribed to pregnant women who will clearly benefit from it; however, its benefits should be weighed against potential risks. For example, one 2007 study found that metronidazole was associated with a greater risk of preterm delivery in high-risk individuals when taken during the second trimester (4th–6th month of pregnancy). 

“The risk of birth defects or miscarriage with metronidazole use during pregnancy is generally considered to be relatively low, especially when used as prescribed,” Dr. Dayananda points out.

Metronidazole comes in various forms, including: 

  • Oral metronidazole capsules (375 mg) and tablets (250 mg or 500 mg)
  • Intravenous forms (5 mg/mL and 500 mg)
  • Topical forms (0.75% and 37.5 mg per applicator), which are applied like vaginal gels

One form or mode of administration may be safer during pregnancy than another. For instance, animal research indicates that “when swallowed, metronidazole was not harmful to baby mice,” says Benjamin Gibson, Pharm.D., adjunct professor at the University of Texas at Austin. On the other hand, injectable forms carry a greater risk in animal studies.

To minimize any adverse effects, it is important to take metronidazole with caution during pregnancy and to stick with the regimen prescribed by a healthcare professional. Notably, it is recommended to take metronidazole only for a short time and to avoid taking it during the first trimester (first three months) of pregnancy to reduce any potential risks. If you are on metronidazole and find out you are pregnant, inform your healthcare provider to determine if you should continue with the medication, stop it, or change to another one instead.

Side effects of metronidazole

While metronidazole is unlikely to cause harm to a fetus, it can cause side effects such as:

  • Upset stomach or cramps
  • Nausea and vomiting
  • Loss of appetite
  • Unpleasant metallic taste
  • Diarrhea
  • Dry mouth

In rare cases, it may cause more severe side effects such as allergic reactions (that may cause symptoms like confusion, rashes, and difficulty breathing), severe stomach pain, and blurred vision. Seek immediate medical attention if you experience such side effects.

What can I take instead of metronidazole during pregnancy?

Depending on your specific infection, stage of pregnancy, and medical history, your healthcare provider may recommend alternative treatments to metronidazole. Alternatives to metronidazole include antibiotics such as:

Additionally, some natural treatments may help certain infections that metronidazole would be used to treat. Bacterial vaginosis is an infection characterized by an alteration in the normal vaginal microbiome that has been associated with premature labor and spontaneous abortions in pregnant patients. Probiotics and prebiotics promote the growth of beneficial microorganisms in the vagina and can be effective for the treatment and prevention of bacterial vaginosis.

The bottom line: Metronidazole is considered safe

Metronidazole is considered safe for use during pregnancy when taken in the second and third trimesters under the guidance of your healthcare provider. In fact, its benefits outweigh the potential risks to the fetus of leaving certain infections untreated. Depending on your infection and stage of pregnancy, your provider may prescribe an alternative medication.

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