What the Research Says
Metronidazole crosses the placenta to go from the parent to the fetus; from there, it enters the fetus’s circulation quickly. In studies, lab animals that received doses up to five times the human dose showed no signs of fertility problems or harm to the fetus. Feeding high doses of metronidazole to pregnant mice did not appear toxic to the fetuses. The FDA does note that, in one small study in which researchers injected metronidazole into the pregnant mice, some lab animal fetuses did die. Researchers have not yet determined the relationship between metronidazole and the deaths in that study.
There are currently no adequate or well-controlled studies investigating the use of metronidazole in pregnant humans. Because drug studies using lab animals do not always predict how humans will respond to medications, and because its effects on human fetuses are unknown, metronidazole should only be used during pregnancy if the patient clearly needs it and if other treatments have not worked. Furthermore, the National Institutes of Health (NIH) says that metronidazole may cause cancer in humans.
Metronidazole and Lactation
Metronidazole can pass from the parent to the child while breastfeeding. The effect of such passing is unknown but it may be of concern. Namely, the results of older studies suggest that this drug could cause genetic mutations in the baby. However, American Family Physician (AAP) says that the amount of metronidazole transferred to the baby through breast milk is too low to cause any harm, and that there have been no reports of adverse effects from exposure to breast milk. Still, AAP recommends that mothers stop breastfeeding for 24 to 48 hours after receiving a 2-gram dose of metronidazole.
Is Metronidazole Safe in the First Trimester of Pregnancy?
Metronidazole may not be safe in the first trimester of pregnancy, as the drug passes through the placental barrier to move from parent to the fetus and is then rapidly absorbed by the fetus. However, researchers are still working to understand if using this antibiotic poses a danger to the parent or the fetus.
Which Antibiotics are Safe During Pregnancy?
Researchers are working to determine if antibiotics are safe during pregnancy.
The FDA categorizes the risk associated with specific drug use during pregnancy as demonstrated through well-controlled scientific studies. Category A drugs being the safest. That is, there is scientific evidence that the drugs cause no harm to the parent or fetus. Category D and Category X drugs are the least safe during pregnancy, with research studies showing that the drugs cause great harm to both parent and fetus.
No antibiotics earn a Category A rating, but several antibiotics are Category B medications. This classification means that studies with lab animals fail to show a risk to the fetus and there are no adequate studies in human women that show the drugs pose a danger to mothers. Category B antibiotics include:
- Clindamycin (Cleocin)
- Penicillin G
- Penicillin V potassium (V-Cillink)
Why is Metronidazole Contraindicated in Pregnancy?
Metronidazole is contraindicated in pregnancy during the first trimester, which means pregnant individuals should not use it because it passes from the parent to the fetus. Furthermore, the fetus’s body absorbs the drug quickly. Furthermore, National Institutes of Health (NIH) says that metronidazole is likely to be a cancer-causing chemical in humans, as high doses of this drug in lab mice resulted in cancer.