In the United Kingdom guidance issued by the medicines information centres and the National Teratology Information Service recommends metronidazole rather than oral vancomycin after a careful risk assessment of each drug.

Vancomycin is poorly absorbed from normal, intact gastrointestinal mucosa, but an inflammatory bowel process can result in increased absorption of the oral product. In patients with pseudomembranous colitis vancomycin may occasionally reach therapeutic concentrations in serum, which can theoretically damage a fetus's VIIIth cranial nerve.

In contrast, several epidemiological studies in women have shown no conclusive evidence that metronidazole causes an increased risk of malformations, stillbirths, or low birth weight.^2–4 The accumulated data on more than 1500 births with prenatal exposure to metronidazole suggests no increase in congenital anomalies.^2–4

In addition, a retrospective cohort study of nearly 1400 exposed pregnancies did not detect an increase in infants with any of several categories of congenital anomaly or low birth weight. Analysis of data from the Hungarian case-control surveillance of congenital abnormalities between 1980 and 1991 found no association with congenital anomalies among 266 pregnancies treated with oral metronidazole during the first trimester. A recent prospective controlled study of 228 women exposed to metronidazole in pregnancy, 86% of whom were exposed in the first trimester, confirmed these findings.

The long term postnatal effects of intrauterine exposure to metronidazole, if any, have yet to be determined. However, data from a 20 year ongoing study give no indication of an increased incidence of malignancies after metronidazole treatment.⁵