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Metronidazole is listed by the US
National Toxicology Program (NTP) as reasonably anticipated to be a human
carcinogen.
[15] Although some of the testing methods have been questioned, oral exposure has been shown to cause cancer in experimental animals and has also demonstrated some mutagenic effects in bacterial cultures.
[15][16] The relationship between exposure to metronidazole and human cancer is unclear.
[15][17] One study
[18][
full citation needed] found an excess in lung cancer among women (even after adjusting for smoking), while other studies
[19][
full citation needed] found either no increased risk, or a
statistically insignificant risk.
[15][20] Metronidazole is listed as a possible carcinogen according to the
WHOInternational Agency for Research on Cancer.
[21] A study in those with
Crohn's diseasealso found chromosomal abnormalities in circulating lymphocytes in people treated with metronidazole.
[16]
Due to its potential carcinogenic properties, metronidazole is banned in the
European Union and the
USA for veterinary use in the feed of animals and is banned for use in any food animals in the US