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>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. (N Engl J Med 301:519–522, 1979) EXPERIMENTAL studies have shown that metronidazole (Flagyl) is both carcinogenic and mutagenic.1 2 3 4 Nevertheless, there have been no long-term studies of human beings exposed to the drug. Metronidazole was introduced into the United States in 1960 for the treatment of trichomoniasis.5 The list of conditions for which the drug is indicated has since grown impressively to include such diverse diseases as amebiasis,6 brain abscess7 and Crohn's disease.8 Roe9 has reviewed the uses and toxicity of metronidazole. Although there is a large and varied clinical experience, the monograph of the International Agency for Research on Cancer, which reviewed data on the carcinogenic. © 1979, Massachusetts Medical Society. All rights reserved.
