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Toxicologic evaluation of metronidazole with particular reference to carcinogenic, mutagenic, and teratogenic
Abstract:
The gastrointestinal tract and nervous system are the main targets for metronidazole toxicity. With the possible exception of certain neurotoxic effects in a few heavily treated patients, all the toxic effects of metronidazole are transient and reversible on withdrawal of the drug. Properly designed tests for embryotoxicity and teratogenicity in rats, rabbits, and mice have produced convincingly negative results, and no adverse effects on the fetus have been observed in women given the drug for trichomoniasis during various stages of pregnancy. The antimicrobial activity of metronidazole is thought to depend on its nitroreduction to form short-lived cytotoxic metabolites capable of reacting with deoxyribonucleic acid. It is therefore perhaps not surprising that metronidazole has been reported to be mutagenic for certain strains of Salmonella typhimurium. Metronidazole gave negative results in the mouse micronucleus test and no increase in sister chromatid exchanges or chromosomal aberrations in cultured human lymphocytes. Prolonged exposure to metronidazole in the treatment of patients with Crohn's disease was not associated with any increase in the frequency of chromosomal aberrations. Prolonged high-dose exposure of mice to metronidazole led to an increased incidence of lung tumors in three separate studies and to a suggestive increase in lymphoreticular neoplasia in female animals in one of the studies. These effects are probably nonspecific, since major effects on the incidence of neoplasms of the same and other kinds have been produced by merely varying the amount of a standard diet that mice consume. A reported excess of liver tumors in rats exposed to metronidazole can be explained by the fact that the authors failed to age standardize their data despite a big and highly significant beneficial effect of the drug on survival. Two carcinogenicity studies in hamsters have given entirely negative results. The follow-up for 10 or more years of 771 women first treated with metronidazole between 1960 and 1969 for trichomoniasis revealed no excess of any form of cancer attributable to treatment, and no excess cancer risk has so far come to light in the Kaiser-Permanente follow-up of nearly 2500 patients given at least one prescription of metronidazole between 1969 and 1973. At the present time it is reasonable to conclude that this highly useful, and sometimes life-saving, drug is essentially free of cancer risk or other serious toxic side effects.
Insights
Metronidazole is generally safe, with most toxic effects being temporary. Extensive studies show no significant risks of birth defects or cancer, making it a valuable medication.
Area of Science:
- Pharmacology
- Toxicology
- Oncology
Background:
- Metronidazole is a widely used antimicrobial agent.
- Its toxicity primarily affects the gastrointestinal and nervous systems.
- Concerns exist regarding its potential mutagenicity and carcinogenicity.
Purpose of the Study:
- To comprehensively evaluate the safety profile of metronidazole.
- To assess potential embryotoxicity, teratogenicity, mutagenicity, and carcinogenicity.
- To determine long-term cancer risks associated with metronidazole use.
Main Methods:
- Review of animal studies (rats, rabbits, mice) for embryotoxicity and teratogenicity.
- In vitro studies using Salmonella typhimurium and human lymphocytes.
- Long-term carcinogenicity bioassays in rodents and hamsters.
- Epidemiological follow-up studies of women treated for trichomoniasis and general patient populations.
Main Results:
- Animal studies showed no evidence of embryotoxicity or teratogenicity.
- Metronidazole demonstrated mutagenicity in some bacterial strains but not in mammalian systems.
- Long-term, high-dose exposure in mice showed some tumor incidence increases, likely nonspecific.
- Extensive human follow-up studies revealed no increased cancer risk attributable to metronidazole.
- Reported rat liver tumor data was confounded by lack of age standardization.
Conclusions:
- Metronidazole exhibits transient and reversible toxic effects, primarily in the GI and nervous systems.
- The drug is not considered embryotoxic, teratogenic, or a significant mutagen in humans.
- Available evidence strongly suggests metronidazole is essentially free of cancer risk.
- Metronidazole is a safe and valuable therapeutic agent.