half-life of norfloxacin in serum and plasma is 3–4 hours. steady-state concentrations of norfloxacin will be attained within two days of dosing. renal excretion occurs by both glomerular filtration and tubular secretion as evidenced by the high rate of renal clearance (approximately 275 ml/min). within 24 hours of drug administration, 26 to 32% of the administered dose is recovered in the urine as norfloxacin with an additional 5-8% being recovered in the urine as six active metabolites of lesser antimicrobial potency. only a small percentage (less than 1%) of the dose is recovered thereafter. fecal recovery accounts for another 30% of the administered dose. two to three hours after a single 400-mg dose, urinary concentrations of 200 μg/ml or more are attained in the urine. in healthy volunteers, mean urinary concentrations of norfloxacin remain above 30 μg/ml for at least 12 hours following a 400-mg dose. the urinary ph may affect the solubility of norfloxacin. norfloxacin is least soluble at urinary ph of 7.5 with greater solubility occurring at phs above and below this value. the serum protein binding of norfloxacin is between 10 and 15%." quoting from the 2009 package insert for noroxin. biotransformation is via the liver and kidneys, with a half-life of 3–4 hours. history the first members of the quinolone antibacterial class were relatively low potency drugs such as nalidixic acid, used mainly in the treatment of urinary tract infections owing to their renal excretion and propensity to be concentrated in urine. in 1979 the publication of a patent filed by the pharmaceutical arm of kyorin seiyaku kabushiki kaisha disclosed the discovery of norfloxacin, and the demonstration that certain structural modifications including the attachment of a fluorine atom to the quinolone ring leads to dramatically enhanced antibacterial potency. in spite of the substantial increase in antibacterial activity of norfloxacin relative to early fluoroquinolones, it did not become a widely used antibiotic. other companies initiated fluoroquinolone discovery programs in the aftermath of the publication of the norfloxacin patent. bayer pharmaceuticals discovered that the addition of a single carbon atom to the norfloxacin structure provided another 4 to 10-fold improvement in activity. ciprofloxacin reached the market just one year after norfloxacin and achieved sales of 1.5 billion euros at its peak. kyorin granted merck & company, inc., an exclusive license (in certain countries, including the united states), to import and distribute norfloxacin under the brand name noroxin. the u.s. food and drug administration (fda) approved noroxin for distribution in the united states on october 31, 1986. availability in most countries, all formulations require a prescription. in colombia (south america) it is marketed under ambigram from laboratorios bussié noroxin was discontinued in the us as of april 2014 see the latest package insert for norfloxacin (noroxin) for additional details.