July 24, 2016




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General: Topical metronidazole formulations have been reported to cause tearing of the eyes. Therefore, contact with the eyes should be avoided. If a reaction suggesting local irritation occurs, patients should be directed to use the medication less frequently or discontinue use. Metronidazole is a nitroim-idazole and should be used with care in patients with evidence or history of blood dyscrasia.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Metronidazole has shown evidence of carcinogenic activity in a number of studies involving chronic, oral administration in mice and rats but not in studies involving hamsters. Neither carcinogenicity nor photocarcinogenicity studies have been performed by the topical route with MetroLotion® Topical Lotion or any marketed metronidazole formulations.

In several long-term studies in mice, oral doses of approximately 198 mg/m2/day or greater (approximately 29 to 71 times the human topical dose on a mg/m2 basis) were associated with an increase in lung tumors in male mice and lymphomas in female mice.

Several long-term studies by the oral route in rats have shown statistically significant increases in mammary and hepatic tumors in female rats and testicular tumors and pituitary adenomas in male rats at doses (in feed) of 1593 mg/m2/day or greater (approximately 230 to 573 times the human topical dose on a mg/m2 basis). In another oral study (by gavage), mammary tumors in female rats were observed with a dose of 177 mg/m2/day (approximately 26 to 64 times the human topical dose on a mg/m2 basis).

In a published study, the ultraviolet radiation-induced carcinogenesis was enhanced in albino hairless mice by intraperitoneal administration of 45 mg/m2 metronidazole, as shown by a decreased latency period to the development of skin neoplasms. The concentration of metronidazole in the skin following the intraperitoneal administration was not determined. This study did not distinguish whether metronidazole must be present during the exposure to ultraviolet radiation in order to enhance tumor formation or whether metronidazole could promote tumor formation from preexisting ultraviolet radiation-initiated cells. The significance of these results in the topical use of metronidazole for the treatment of rosacea is unclear.

Metronidazole has shown evidence of mutagenic activity in several in vitro bacterial assay systems. In addition, a dose-response increase in the frequency of micronuclei was observed in mice after intraperitoneal injections. An increase in chromosome aberrations in peripheral blood lymphocytes was reported in patients with Crohn's disease who were treated with 200-1200 mg/day of metronidazole for 1 to 24 months. However, in another study, no excess chromosomal aberrations in circulating human lymphocytes were observed in patients treated for 8 months.

In rats, oral metronidazole at doses of 1770 mg/m2/day (approximately 255 to 637 times the human topical dose on a mg/m2 basis) induced inhibition of spermatogenesis and severe testicular degeneration. In two strains of mice (ICR and CF1), conflicting results have been reported indicating either no effect or a similar effect to that reported in rats.


Teratogenic Effects: Pregnancy Category B

There are no adequate and well-controlled studies with the use of MetroLotion® Topical Lotion in pregnant women. Metronidazole crosses the placental barrier and enters the fetal circulation rapidly. No fetotoxicity was observed after oral administration of metronidazole in rats or mice. However, because animal reproduction studies are not always predictive of human response and since oral metronidazole has been shown to be a carcinogen in some rodents, this drug should be used during pregnancy only if clearly needed.

Nursing Mothers

After oral administration, metronidazole is secreted in breast milk in concentrations similar to those found in the plasma. Even though blood levels are significantly lower with topically applied metronida-zole than those achieved after oral administration of metronidazole, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

Pediatric Use

Safety and effectiveness in pediatric patients have not been established.

This monograph has been modified to include the generic and brand name in many instances.

Last reviewed on RxList: 9/3/2008


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