Lack of evidence for cancer due to use of metronidazole

Experimental studies have shown that metronidazole is carcinogenic in rodents and mutagenic in bacteria. In 771 women given metronidazole for the treatment of vaginal trichomoniasis, more cancers developed than had been expected after exclusion of carcinoma of one uterine cervix (observed, 24 cases; expected on the basis of the Connecticut Tumor Registry, 21.7; expected on the basis of the Third National Cancer Survey, 18.4). However, the excess was not statistically significant (P greater than 0.05). The observed and expected numbers of breast-cancer cases were the same, but four lung-cancer cases were observed, whereas 0.6 would have been expected. This finding is confounded by the fact that all four lung cancers developed in women who were smokers. Overall, we observed no appreciable increase in cases of cancer.