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Discontinued Warning IconPlease Note: This Brand Name drug is no longer available in the US.
(Generic versions may still be available.)



Norfloxacin has in vitro activity against a broad spectrum of gram-positive and gram-negative aerobic bacteria. The fluorine atom at the 6 position provides increased potency against gram-negative organisms and the piperazine moiety at the 7 position is responsible for anti-pseudomonal activity.

Norfloxacin inhibits bacterial deoxyribonucleic acid synthesis and is bactericidal. At the molecular level three specific events are attributed to CHIBROXIN (norfloxacin) in E. coli cells:

  1. inhibition of the ATP-dependent DNA supercoiling reaction catalyzed by DNA gyrase;
  2. inhibition of the relaxation of supercoiled DNA;
  3. promotion of double-stranded DNA breakage.

There is generally no cross-resistance between norfloxacin and other classes of antibacterial agents. Therefore, norfloxacin generally demonstrates activity against indicated organisms resistant to some other antimicrobial agents. When such cross-resistance does occur, it is probably due to decreased entry of the drugs into the bacterial cells. Antagonism has been demonstrated in vitro between norfloxacin and nitrofurantoin.

Norfloxacin has been shown to be active against most strains of the following organisms both in vitro and clinically in ophthalmic infections (see INDICATIONS AND USAGE ):

Gram-positive bacteria including:

Staphylococcus aureus

Staphylococcus epidermidis

Staphylococcus warnerii

Streptococcus pneumoniae

Gram-negative bacteria including:

Acinetobacter calcoaceticus

Aeromonas hydrophila

Haemophilus influenzae

Proteus mirabilis

Pseudomonas aeruginosa

Serratia marcescens

Norfloxacin has been shown to be active in vitro against most strains of the following organisms; however, the clinical significance of these data in ophthalmic infections is unknown.

Gram-positive bacteria:

Bacillus cereus

Enterococcus faecalis (formerly Streptococcus faecalis  )

Staphylococcus saprophyticus

Gram-negative bacteria:

Citrobacter diversus

Citrobacter freundii

Edwardsiella tarda

Enterobacter aerogenes

Enterobacter cloacae

Escherichia coli

Hafnia alvei

Haemophilus aegyptius (Koch-Weeks bacillus)

Klebsiella oxytoca

Klebsiella pneumoniae

Klebsiella rhinoscleromatis

Morganella morganii

Neisseria gonorrhoeae

Proteus vulgaris

Providencia alcalifaciens

Providencia rettgeri

Providencia stuartii

Salmonella typhi

Vibrio cholerae

Vibrio parahemolyticus

Yersinia enterocolitica


Ureaplasma urealyticum

Norfloxacin is not active against obligate anaerobes.

Clinical Studies

Clinical studies were conducted comparing CHIBROXIN (norfloxacin) Ophthalmic Solution (n=152) with ophthalmic solutions of tobramycin, gentamicin, and chloramphenicol (n=158) in patients with conjunctivitis and positive bacterial cultures. After seven days of therapy with CHIBROXIN (norfloxacin) Ophthalmic Solution, 72 percent of patients were clinically cured. Of those cured, 85 percent had all their pathogens eradicated. Eradication was also achieved in 62 percent (23/37) of patients whose clinical outcome was not completely cured by day seven. These results were similar among all treatment groups.

Another clinical study compared CHIBROXIN (norfloxacin) Ophthalmic Solution with placebo in patients with conjunctivitis and positive bacterial cultures. Placebo in this study was the liquid vehicle for CHIBROXIN (norfloxacin) Ophthalmic Solution and contained the preservative. After five days of therapy, 64 percent (36/56) of patients on CHIBROXIN (norfloxacin) Ophthalmic Solution were clinically cured compared to 50 percent (23/46) of patients receiving placebo. Of those cured, 78 percent had all their pathogens eradicated. Eradication was also achieved in 50 percent (10/20) of patients whose clinical outcome was not completely cured. The response to CHIBROXIN (norfloxacin) Ophthalmic Solution was statistically significantly better than the response to placebo.


The oral administration of single doses of norfloxacin, six times the recommended human oral dose***, caused lameness in immature dogs. Histologic examination of the weight-bearing joints of these dogs revealed permanent lesions of the cartilage. Related drugs also produced erosions of the cartilage in weight-bearing joints and other signs of arthropathy in immature animals of various species.

***All factors are based on a standard patient weight of 50 kg. The usual oral dose of norfloxacin is 800 mg daily. One drop of CHIBROXIN (norfloxacin) Ophthalmic Solution 0.3% contains about 1/6,666 of this dose (0.12 mg).

Last reviewed on RxList: 12/8/2004
This monograph has been modified to include the generic and brand name in many instances.

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