I've been reading more and more about the toxic effects of Flagyl in some cats as reported on various Yahoo feline support groups. I found this article to be the best one posted to date that covers all aspects of "Metronidazole Neurotoxicosis." The neurotoxicosis is being reported in cats when used for as little as 40 days.
I know that vets routinely prescribe Flagyl for IBD kitties with diarrhea and constipation both. It appears as though a regiment of Flagyl for well under 40 days would be advisable to be on the safe side.
This publication is dated 2005 and I'm surprised at how many vets still do not take into consideration the recommended term the drug should be used for. I have used Flagyl on one of my seniors earlier this year and it was 30-40 days on the drug at times without any warnings from the vets.http://www.vetpathology.org/cgi/reprint/42/5/665.pdf