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Hi,

I was wondering if anyone knew how flagyl builds up in the body over each day. For example, how long (how many days) does it take for the flagyl to reach its peak levels in our blood?

I am always careful what I believe since theres plenty of incorrect information even in the peer-reviewed literature. That said,

http://www.rxlist.com/cgi/generic/metronidaz_cp.htm

Disposition of metronidazole in the body is similar for both oral and intravenous dosage forms, with an average elimination half-life in healthy humans of eight hours.

Elimination half-life is often just called "t(1/2)", and the (fairly) constant blood level you refer to is usually called "steady state." At steady state your body level still goes up and down between doses, but reaches about the same peak and the same trough every time.

With a t(1/2) of 8 hours you should hit steady state in just a couple days. In other words, after a couple days the magnitudes of your body level peaks and troughs are within ~15% of the ideal steady state ones.

Speak of the devil, I'm doing a lil nitroimidazole research today and stumbled over this:

Metronidazole is extensively metabolised by the liver to 5 metabolites. The hydroxy metabolite has biological activity of 30 to 65% and a longer elimination half-life than the parent compound.

PMID: 10384859

So, its not just about how much flagyl is in you; this flagyl metabolite is also active. By "biological activity" I would assume they probably(?) mean antibacterial activity. Because it has a longer t(1/2), the metabolite will also take longer to converge to the steady state concentration curve.

Not all the flagyl you ingest will be changed to the hydroxy metabolites; some will be changed to other metabolites, and some will be excreted unchanged.