Metronidazole is an antibiotic most often used to treat rosacea, trichomoniasis and bacterial vaginosis. The U.S. Food and Drug Administration (FDA) classifies it in Pregnancy Category B, which normally means animal studies have shown no ill effects on fetuses. This drug, however, has shown evidence that it increases the risk of cancer when used in pregnant rats, which is why pregnant women are often advised to avoid metronidazole in pregnancy when possible. The conflicting evidence means doctors may prescribe metronidazole in pregnancy only when they feel it is the one medication that can get rid of certain bacterial infections, but they tend to at least wait until the first trimester has passed.
This drug has been placed in Pregnancy Category B because studies involving animals do not provide evidence of any harm coming to the mother or fetus. They have, however, found that this drug can cause a miscarriage when injected straight into the abdomen of pregnant animals. In addition, it seems this drug can act as a carcinogen, because rats and mice that were given metronidazole in pregnancy were more likely to develop cancer than those that did not take the drug. While humans and animals often react differently to medication, no studies have been completed to show whether this drug increases the risk of cancer in humans, which is why it should be used with caution.
Even though metronidazole in pregnancy may be prescribed by doctors who believe it to be the only treatment for the problem, it is not typically supposed to be taken in the first trimester. When it is used later in pregnancy, it is often broken up into small doses, because taking one large dose may be harmful. Breastfeeding mothers also are discouraged from taking the drug, because some of it ends up in the breast milk, and the effects on the baby are not known as of 2011.
In general, doctors usually only prescribe metronidazole in pregnancy when the benefits outweigh the risks. For example, bacterial vaginosis can lead to preterm labor and low birth weight when it is not treated, so doctors may prescribe metronidazole to get rid of the infection and the associated risks. Trichomoniasis also may cause low birth weight, preterm labor and premature rupture of membranes, but evidence shows that treatment during pregnancy does not necessarily reduce these risks and may even increase them. For this reason, it may only be treated when symptoms are extreme. Similarly, using metronidazole in pregnancy to treat rosacea may not be appropriate unless the case is severe, because this condition rarely causes long-term harm to the mother or fetus, meaning treatment can often wait until the baby is born.