This post was written by Laurie Goldstein.
Is your kitty on the antibiotic metronadizole (also called flagyl)? This is not intended to scare people, but it’s important information that needs to be carefully considered… metronadizole is potentially carcinogenic both to humans and cats. From a 2009 study in cats:
“Metronidazole kills target organisms by inducing formation of reactive intermediates within these organisms, resulting in disruption of DNA. This disruptive effect is not restricted to microorganisms, and both carcinogenic and mutagenic effects have been documented in experimental animals and metronidazole disrupts DNA in human blood lymphocytes. The importance of these disruptions is unclear in humans – damage was not evident within 6 days of discontinuation of the drug. No information about genotoxicity in cats is available.”
The study, “Single-dose pharmacokinetics and genotoxicity of metronidazole in cats” (Sekis et al. 2009. Journal of Feline Medicine and Surgery (2009) 11, 60-68) found:
“Genotoxicity, as measured by DNA disruption in PBMC obtained from cats administered metronidazole for 7 days and a feline lymphocyte cell line incubated with metronidazole, was observed in all cats and all lymphocyte experiments. This is similar to observations in people treated with metronidazole. The DNA damage resolved within 6 days of discontinuing metronidazole, also similar to findings in humans, suggesting that DNA repair mechanisms correct the disruption induced by metronidazole. However, experimental studies in rodents have demonstrated both carcinogenic and mutagenic effects of metronidazole. Lymphoma is the most common hematological neoplasia in cats and cats have a propensity to develop intestinal T-cell lymphoma. It is interesting to speculate that chronic metronidazole therapy may damage lymphoid DNA, which, if unable to be effectively repaired by a defective repair system, could result in development of lymphoma. This may be particularly relevant to cats receiving chronic metronidazole to treat inflammatory bowel disease. Currently, no evidence exists that chronic metronidazole therapy results in lymphoma formation in cats, and additional associative studies are needed to show a correlation between chronic metronidazole exposure and subsequent development of lymphoma.”
Bold, my emphasis.
Also of note is a 2002 study, “Is metronadizole carcinogenic?” Bendesky et al., Mutation Research 511 (2002) 133–144.
“There is sufficient evidence to consider MTZ as an animal carcinogen. In this regard, the IARC has classified MTZ as an animal carcinogen. Germany has already prohibited MTZ for veterinary use while further studies are being carried out.”
Importantly, when used for 7 days, the DNA damage resolved in 6 days. So short term use, as when treating for Giardia, is likely fine. But so many vets throw metro at just about any diarrhea, and cats suffering from Inflammatory Bowel Disease (IBD) are often put on it for long periods of time. If your vet prescribes metro, you should thoroughly discuss the risks vs. rewards of using this drug before you fill that prescription.
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