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Mechanism Of Action

Ciprofloxacin is a member of the fluoroquinolone class of antibacterial agents [see Microbiology].



The absolute bioavailability of ciprofloxacin when given as an oral tablet is approximately 70% with no substantial loss by first pass metabolism. Ciprofloxacin maximum serum concentrations and area under the curve are shown in the chart for the 250 mg to 1000 mg dose range (Table 10).

Table 10: Maximum Serum Concentrations and Areas Under the Curve

Dose (mg) Maximum Serum Concentration (mcg/mL) Area Under Curve (AUC) (mcg•hr/mL)
250 1.2 4.8
500 2.4 11.6
750 4.3 20.2
1000 5.4 30.8

Maximum serum concentrations are attained 1 to 2 hours after oral dosing. Mean concentrations 12 hours after dosing with 250, 500, or 750 mg are 0.1, 0.2, and 0.4 mcg/mL, respectively. The serum elimination half-life in subjects with normal renal function is approximately 4 hours. Serum concentrations increase proportionately with doses up to 1000 mg.

A 500 mg oral dose given every 12 hours has been shown to produce an area under the serum concentration time curve (AUC) equivalent to that produced by an intravenous infusion of 400 mg CIPRO given over 60 minutes every 12 hours. A 750 mg oral dose given every 12 hours has been shown to produce an AUC at steady-state equivalent to that produced by an intravenous infusion of 400 mg given over 60 minutes every 8 hours. A 750 mg oral dose results in a Cmax similar to that observed with a 400 mg intravenous dose. A 250 mg oral dose given every 12 hours produces an AUC equivalent to that produced by an infusion of 200 mg CIPRO given every 12 hours (Table 11).

Table 11: Steady-state Pharmacokinetic Parameters Following Multiple Oral and IV Doses

Parameters 500 mg 400 mg 750 mg 400 mg
  every 12 hours, orally every 12 hours, intravenous every 12 hours, orally. every 8 hours, intravenous
AUC (mcg•hr/mL) 13.71 12.71 31.62 32.93
Cmax (mcg/mL) 2.97 4.56 3.59 4.07
AUC 0–12h
2AUC 24h = AUC 0–12h x 2
3AUC 24h = AUC 0–8h x3


When CIPRO Tablet is given concomitantly with food, there is a delay in the absorption of the drug, resulting in peak concentrations that occur closer to 2 hours after dosing rather than 1 hour whereas there is no delay observed when CIPRO Suspension is given with food. The overall absorption of CIPRO Tablet or CIPRO Suspension, however, is not substantially affected. The pharmacokinetics of ciprofloxacin given as the suspension are also not affected by food. Avoid concomitant administration of CIPRO with dairy products (like milk or yogurt) or calcium-fortified juices alone since decreased absorption is possible; however, CIPRO may be taken with a meal that contains these products

With oral administration, a 500 mg dose, given as 10 mL of the 5% CIPRO Suspension (containing 250 mg ciprofloxacin/5mL) is bioequivalent to the 500 mg tablet. A 10 mL volume of the 5% CIPRO Suspension (containing 250 mg ciprofloxacin/5mL) is bioequivalent to a 5 mL volume of the 10% CIPRO Suspension (containing 500 mg ciprofloxacin/5mL).


The binding of ciprofloxacin to serum proteins is 20% to 40% which is not likely to be high enough to cause significant protein binding interactions with other drugs.

After oral administration, ciprofloxacin is widely distributed throughout the body. Tissue concentrations often exceed serum concentrations in both men and women, particularly in genital tissue including the prostate. Ciprofloxacin is present in active form in the saliva, nasal and bronchial secretions, mucosa of the sinuses, sputum, skin blister fluid, lymph, peritoneal fluid, bile, and prostatic secretions. Ciprofloxacin has also been detected in lung, skin, fat, muscle, cartilage, and bone. The drug diffuses into the cerebrospinal fluid (CSF); however, CSF concentrations are generally less than 10% of peak serum concentrations. Low levels of the drug have been detected in the aqueous and vitreous humors of the eye.


Four metabolites have been identified in human urine which together account for approximately 15% of an oral dose. The metabolites have antimicrobial activity, but are less active than unchanged ciprofloxacin. Ciprofloxacin is an inhibitor of human cytochrome P450 1A2 (CYP1A2) mediated metabolism. Co-administration of ciprofloxacin with other drugs primarily metabolized by CYP1A2 results in increased plasma concentrations of these drugs and could lead to clinically significant adverse events of the co-administered drug [see CONTRAINDICATIONS, WARNINGS AND PRECAUTIONS, and DRUG INTERACTIONS].


The serum elimination half-life in subjects with normal renal function is approximately 4 hours. Approximately 40 to 50% of an orally administered dose is excreted in the urine as unchanged drug. After a 250 mg oral dose, urine concentrations of ciprofloxacin usually exceed 200 mcg/mL during the first two hours and are approximately 30 mcg/mL at 8 to 12 hours after dosing. The urinary excretion of ciprofloxacin is virtually complete within 24 hours after dosing. The renal clearance of ciprofloxacin, which is approximately 300 mL/minute, exceeds the normal glomerular filtration rate of 120 mL/minute. Thus, active tubular secretion would seem to play a significant role in its elimination. Co-administration of probenecid with ciprofloxacin results in about a 50% reduction in the ciprofloxacin renal clearance and a 50% increase in its concentration in the systemic circulation.

Although bile concentrations of ciprofloxacin are several fold higher than serum concentrations after oral dosing, only a small amount of the dose administered is recovered from the bile as unchanged drug. An additional 1% to 2% of the dose is recovered from the bile in the form of metabolites. Approximately 20% to 35% of an oral dose is recovered from the feces within 5 days after dosing. This may arise from either biliary clearance or transintestinal elimination.

Specific Populations


Pharmacokinetic studies of the oral (single dose) and intravenous (single and multiple dose) forms of ciprofloxacin indicate that plasma concentrations of ciprofloxacin are higher in elderly subjects (older than 65 years) as compared to young adults. Although the Cmax is increased 16% to 40%, the increase in mean AUC is approximately 30%, and can be at least partially attributed to decreased renal clearance in the elderly. Elimination half-life is only slightly (~20%) prolonged in the elderly. These differences are not considered clinically significant [see Use In Specific Populations].

Renal Impairment

In patients with reduced renal function, the half-life of ciprofloxacin is slightly prolonged. Dosage adjustments may be required [see Use in Specific Populations and DOSAGE AND ADMINISTRATION].

Hepatic Impairment

In preliminary studies in patients with stable chronic liver cirrhosis, no significant changes in ciprofloxacin pharmacokinetics have been observed. The kinetics of ciprofloxacin in patients with acute hepatic insufficiency, have not been fully studied.


Following a single oral dose of 10 mg/kg CIPRO suspension to 16 children ranging in age from 4 months to 7 years, the mean Cmax was 2.4 mcg/mL (range: 1.5 mcg/mL to 3.4 mcg/mL) and the mean AUC was 9.2 mcg*hr/mL (range: 5.8 mcg*hr/mL to 14.9 mcg*h/mL). There was no apparent age-dependence, and no notable increase in Cmax or AUC upon multiple dosing (10 mg/kg three times a day). In children with severe sepsis who were given CIPRO IV (10 mg/kg as a 1-hour intravenous infusion), the mean Cmax was 6.1 mcg/mL (range: 4.6 mcg/mL to 8.3 mcg/mL) in 10 children less than 1 year of age; and 7.2 mcg/mL (range: 4.7 mcg/mL to 11.8 mcg/mL) in 10 children between 1 year and 5 years of age. The AUC values were 17.4 mcg*hr/mL (range: 11.8 mcg*hr/mL to 32 mcg*hr/mL) and 16.5 mcg*hr/mL (range: 11 mcg*hr/mL to 23.8 mcg*hr/mL) in the respective age groups. These values are within the range reported for adults at therapeutic doses. Based on population pharmacokinetic analysis of pediatric patients with various infections, the predicted mean half-life in children is approximately 4 hours –5 hours, and the bioavailability of the oral suspension is approximately 60%.

Drug-Drug Interactions


Concurrent administration of antacids containing magnesium hydroxide or aluminum hydroxide may reduce the bioavailability of ciprofloxacin by as much as 90% [see DOSAGE AND ADMINISTRATION and DRUG INTERACTIONS].

Histamine H2-receptor Antagonists

Histamine H2-receptor antagonists appear to have no significant effect on the bioavailability of ciprofloxacin.


The serum concentrations of ciprofloxacin and metronidazole were not altered when these two drugs were given concomitantly.


In a pharmacokinetic study, systemic exposure of tizanidine (4 mg single dose) was significantly increased (Cmax 7-fold, AUC 10-fold) when the drug was given concomitantly with CIPRO (500 mg twice a day for 3 days). Concomitant administration of tizanidine and CIPRO is contraindicated due to the potentiation of hypotensive and sedative effects of tizanidine [see CONTRAINDICATIONS].


In a study conducted in 12 patients with Parkinson's disease who were administered 6 mg ropinirole once daily with 500 mg CIPRO twice-daily, the mean Cmax and mean AUC of ropinirole were increased by 60% and 84%, respectively. Monitoring for ropinirole-related adverse reactions and appropriate dose adjustment of ropinirole is recommended during and shortly after co-administration with CIPRO [see WARNINGS AND PRECAUTIONS].


Following concomitant administration of 250 mg CIPRO with 304 mg clozapine for 7 days, serum concentrations of clozapine and N-desmethylclozapine were increased by 29% and 31%, respectively. Careful monitoring of clozapine associated adverse reactions and appropriate adjustment of clozapine dosage during and shortly after co-administration with CIPRO are advised.


Following concomitant administration of a single oral dose of 50 mg sildenafil with 500 mg CIPRO to healthy subjects, the mean Cmax and mean AUC of sildenafil were both increased approximately two-fold. Use sildenafil with caution when co-administered with CIPRO due to the expected two-fold increase in the exposure of sildenafil upon co-administration of CIPRO.


In clinical studies it was demonstrated that concomitant use of duloxetine with strong inhibitors of the CYP450 1A2 isozyme such as fluvoxamine, may result in a 5-fold increase in mean AUC and a 2.5-fold increase in mean Cmax of duloxetine.


In a study conducted in 9 healthy volunteers, concomitant use of 1.5 mg/kg IV lidocaine with CIPRO 500 mg twice daily resulted in an increase of lidocaine Cmax and AUC by 12% and 26%, respectively. Although lidocaine treatment was well tolerated at this elevated exposure, a possible interaction with CIPRO and an increase in adverse reactions related to lidocaine may occur upon concomitant administration.


Metoclopramide significantly accelerates the absorption of oral ciprofloxacin resulting in a shorter time to reach maximum plasma concentrations. No significant effect was observed on the bioavailability of ciprofloxacin.


When CIPRO was administered as a single 1000 mg dose concomitantly with omeprazole (40 mg once daily for three days) to 18 healthy volunteers, the mean AUC and Cmax of ciprofloxacin were reduced by 20% and 23%, respectively. The clinical significance of this interaction has not been determined.


Mechanism Of Action

The bactericidal action of ciprofloxacin results from inhibition of the enzymes topoisomerase II (DNA gyrase) and topoisomerase IV (both Type II topoisomerases), which are required for bacterial DNA replication, transcription, repair, and recombination.

Mechanism Of Resistance

The mechanism of action of fluoroquinolones, including ciprofloxacin, is different from that of penicillins, cephalosporins, aminoglycosides, macrolides, and tetracyclines; therefore, microorganisms resistant to these classes of drugs may be susceptible to ciprofloxacin. Resistance to fluoroquinolones occurs primarily by either mutations in the DNA gyrases, decreased outer membrane permeability, or drug efflux. In vitro resistance to ciprofloxacin develops slowly by multiple step mutations. Resistance to ciprofloxacin due to spontaneous mutations occurs at a general frequency of between < 10-9 to 1x10-6 .

Cross Resistance

There is no known cross-resistance between ciprofloxacin and other classes of antimicrobials.

Ciprofloxacin has been shown to be active against most isolates of the following bacteria, both in vitro and in clinical infections [see INDICATIONS AND USAGE].

Gram-Positive Bacteria

Bacillus anthracis
Enterococcus faecalis

Staphylococcus aureus
(methicillin-susceptible isolates only)
Staphylococcus epidermidis
(methicillin-susceptible isolates only)
Staphylococcus saprophyticus

Streptococcus pneumoniae

Streptococcus pyogenes

Gram-Negative Bacteria

Campylobacter jejuni
Citrobacter koseri

Citrobacter freundii

Enterobacter cloacae

Escherichia coli

Haemophilus influenzae

Haemophilus parainfluenzae

Klebsiella pneumoniae

Moraxella catarrhalis

Morganella morganii

Neisseria gonorrhoeae

Proteus mirabilis

Proteus vulgaris

Providencia rettgeri

Providencia stuartii

Pseudomonas aeruginosa

Salmonella typhi

Serratia marcescens

Shigella boydii

Shigella dysenteriae

Shigella flexneri

Shigella sonnei

Yersinia pestis

The following in vitro data are available, but their clinical significance is unknown. At least 90 percent of the following bacteria exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for ciprofloxacin ( ≤ 1 mcg/mL). However, the efficacy of ciprofloxacin in treating clinical infections due to these bacteria has not been established in adequate and well-controlled clinical trials.

Gram-Positive Bacteria

Staphylococcus haemolyticus (methicillin-susceptible isolates only)
Staphylococcus hominis (methicillin-susceptible isolates only)

Gram-Negative Bacteria

Acinetobacter lwoffi
Aeromonas hydrophila

Edwardsiella tarda

Enterobacter aerogenes

Klebsiella oxytoca

Legionella pneumophila

Pasteurella multocida

Salmonella enteritidis

Vibrio cholerae

Vibrio parahaemolyticus

Vibrio vulnificus

Yersinia enterocolitica

Susceptibility Test

Methods When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drug products used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method (broth and/or agar).5,6,7 The MIC values should be interpreted according to criteria provided in Table 12.

Diffusion Techniques

Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized test method.6,7,8 This procedure uses paper disks impregnated with 5 mcg ciprofloxacin to test the susceptibility of bacteria to ciprofloxacin. The disc diffusion interpretive criteria are provided in Table 12.

Table 12: Susceptibility Test Interpretive Criteria for Ciprofloxacin

Bacteria MIC (mcg/mL) Zone Diameter (mm)
Enterobacteriaceae ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Enterococcus faecalis ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Staphylococcus aureus ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Staphylococcus epidermidis ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Staphylococcus saprophyticus ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Pseudomonas aeruginosa ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Haemophilus influenzae1 ≤ 1 - - ≥ 21 - -
Haemophilus parainfluenzae1 ≤ 1 - - ≥ 21 - -
Salmonella typhi ≤ 0.06 0.12-0.5 ≥ 1 ≥ 31 21-30 ≤ 20
Streptococcus pneumoniae ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Streptococcus pyogenes ≤ 1 2 ≥ 4 ≥ 21 16-20 ≤ 15
Neisseria gonorrhoeae2 ≤ 0.06 0.12-0.5 ≥ 1 ≥ 41 28-40 ≤ 27
Bacillus anthracis1 ≤ 0.25 - - - - -
Yersinia pestis1 ≤ 0.25 - - - - -
S=Susceptible, I=Intermediate, and R=Resistant.
1The current absence of data on resistant isolates precludes defining any results other than “Susceptible.” If isolates yield MIC results other than susceptible, they should be submitted to a reference laboratory for further testing.
2MIC is determined by the agar dilution method

A report of “Susceptible” indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the site of infection necessary to inhibit growth of the pathogen. A report of “Intermediate” indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test.5,6,7,8 Standard ciprofloxacin powder should provide the following range of MIC values noted in Table 13. For the diffusion technique using the ciprofloxacin 5 mcg disk the criteria in Table 13 should be achieved.

Table 13: Acceptable Quality Control Ranges for Ciprofloxacin

Bacteria MIC range (mcg/mL) Zone Diameter (mm)
Enterococcus faecalis ATCC 29212 0.25-2 -
Escherichia coli ATCC 25922 0.004-0.015 30-40
Haemophilus influenzae ATCC 49247 0.004-0.03 34-42
Pseudomonas aeruginosa ATCC 27853 0.25-1 25-33
Staphylococcus aureus ATCC 29213 0.12-0.5 -
Staphylococcus aureus ATCC 25923 - 22-30
Neisseria gonorrhoeae ATCC 492261 0.001-0.008 48-58
Campylobacter jejuni ATCC 33560 0.06-0.25 and 0.03-0.12 -
1MIC is determined by the agar dilution method

Animal Toxicology And/Or Pharmacology

Ciprofloxacin and other quinolones have been shown to cause arthropathy in immature animals of most species tested [see WARNINGS AND PRECAUTIONS]. Damage of weight bearing joints was observed in juvenile dogs and rats. In young beagles, 100 mg/kg ciprofloxacin, given daily for 4 weeks, caused degenerative articular changes of the knee joint. At 30 mg/kg, the effect on the joint was minimal. In a subsequent study in young beagle dogs, oral ciprofloxacin doses of 30 mg/kg and 90 mg/kg ciprofloxacin (approximately 1.3-times and 3.5-times the pediatric dose based upon comparative plasma AUCs) given daily for 2 weeks caused articular changes which were still observed by histopathology after a treatment-free period of 5 months. At 10 mg/kg (approximately 0.6-times the pediatric dose based upon comparative plasma AUCs), no effects on joints were observed. This dose was also not associated with arthrotoxicity after an additional treatment-free period of 5 months. In another study, removal of weight bearing from the joint reduced the lesions but did not totally prevent them.

Crystalluria, sometimes associated with secondary nephropathy, occurs in laboratory animals dosed with ciprofloxacin. This is primarily related to the reduced solubility of ciprofloxacin under alkaline conditions, which predominate in the urine of test animals; in man, crystalluria is rare since human urine is typically acidic. In rhesus monkeys, crystalluria without nephropathy was noted after single oral doses as low as 5 mg/kg. (approximately 0.07-times the highest recommended therapeutic dose based upon body surface area). After 6 months of intravenous dosing at 10 mg/kg/day, no nephropathological changes were noted; however, nephropathy was observed after dosing at 20 mg/kg/day for the same duration (approximately 0.2-times the highest recommended therapeutic dose based upon body surface area).

In dogs, ciprofloxacin at 3 mg/kg and 10 mg/kg by rapid intravenous injection (15 sec.) produces pronounced hypotensive effects. These effects are considered to be related to histamine release, since they are partially antagonized by pyrilamine, an antihistamine. In rhesus monkeys, rapid intravenous injection also produces hypotension but the effect in this species is inconsistent and less pronounced.

In mice, concomitant administration of nonsteroidal anti-inflammatory drugs such as phenylbutazone and indomethacin with quinolones has been reported to enhance the CNS stimulatory effect of quinolones.

Ocular toxicity seen with some related drugs has not been observed in ciprofloxacin-treated animals

Clinical Studies

Complicated Urinary Tract Infection And Pyelonephritis–Efficacy In Pediatric Patients

CIPRO administered intravenously and/or orally was compared to a cephalosporin for treatment of cUTI and pyelonephritis in pediatric patients 1 to 17 years of age (mean age of 6 ± 4 years). The trial was conducted in the US, Canada, Argentina, Peru, Costa Rica, Mexico, South Africa, and Germany. The duration of therapy was 10 to 21 days (mean duration of treatment was 11 days with a range of 1 to 88 days). The primary objective of the study was to assess musculoskeletal and neurological safety.

Patients were evaluated for clinical success and bacteriological eradication of the baseline organism(s) with no new infection or superinfection at 5 to 9 days post-therapy (Test of Cure or TOC). The Per Protocol population had a causative organism(s) with protocol specified colony count(s) at baseline, no protocol violation, and no premature discontinuation or loss to follow-up (among other criteria).

The clinical success and bacteriologic eradication rates in the Per Protocol population were similar between CIPRO and the comparator group as shown below.

Table 14: Clinical Success and Bacteriologic Eradication at Test of Cure (5 to 9 Days Post-Therapy)

  CIPRO Comparator
Randomized Patients 337 352
Per Protocol Patients 211 231
Clinical Response at 5 to 9 Days Post-Treatment 95.7% (202/211) 92.6% (214/231)
  95% CI [-1.3%, 7.3%]
Bacteriologic Eradication by Patient at 5 to 9 Days Post-Treatment1 84.4% (178/211) 78.3% (181/231)
  95% CI [-1 .3%, 13.1%]
Bacteriologic Eradication of the Baseline Pathogen at 5 to 9 Days Post-Treatment  
Escherichia coli 156/178 (88%) 161/179 (90%)
1 Patients with baseline pathogen(s) eradicated and no new infections or superinfections/total number of patients. There were 5.5% (6/211) ciprofloxacin and 9.5% (22/231) comparator patients with superinfections or new infections.

Inhalational Anthrax In Adults And Pediatrics

The mean serum concentrations of ciprofloxacin associated with a statistically significant improvement in survival in the rhesus monkey model of inhalational anthrax are reached or exceeded in adult and pediatric patients receiving oral and intravenous regimens. Ciprofloxacin pharmacokinetics have been evaluated in various human populations. The mean peak serum concentration achieved at steady-state in human adults receiving 500 mg orally every 12 hours is 2.97 mcg/mL, and 4.56 mcg/mL following 400 mg intravenously every 12 hours. The mean trough serum concentration at steady-state for both of these regimens is 0.2 mcg/mL. In a study of 10 pediatric patients between 6 and 16 years of age, the mean peak plasma concentration achieved is 8.3 mcg/mL and trough concentrations range from 0.09 mcg/mL to 0.26 mcg/mL, following two 30-minute intravenous infusions of 10 mg/kg administered 12 hours apart. After the second intravenous infusion patients switched to 15 mg/kg orally every 12 hours achieve a mean peak concentration of 3.6 mcg/mL after the initial oral dose. Long-term safety data, including effects on cartilage, following the administration of CIPRO to pediatric patients are limited. Ciprofloxacin serum concentrations achieved in humans serve as a surrogate endpoint reasonably likely to predict clinical benefit and provide the basis for this indication.1

A placebo-controlled animal study in rhesus monkeys exposed to an inhaled mean dose of 11 LD50 (~5.5 x 105 spores (range 5–30 LD50) of B. anthracis was conducted. The minimal inhibitory concentration (MIC) of ciprofloxacin for the anthrax strain used in this study was 0.08 mcg/mL. In the animals studied, mean serum concentrations of ciprofloxacin achieved at expected Tmax (1 hour post-dose) following oral dosing to steady-state ranged from 0.98 mcg/mL to 1.69 mcg/mL. Mean steady-state trough concentrations at 12 hours post-dose ranged from 0.12 mcg/mL to 0.19 mcg/mL.10 Mortality due to anthrax for animals that received a 30-day regimen of oral ciprofloxacin beginning 24 hours post-exposure was significantly lower (1/9), compared to the placebo group (9/10) [p= 0.001]. The one CIPRO-treated animal that died of anthrax did so following the 30-day drug administration period.11

More than 9300 persons were recommended to complete a minimum of 60 days of antibacterial prophylaxis against possible inhalational exposure to B. anthracis during 2001. CIPRO was recommended to most of those individuals for all or part of the prophylaxis regimen. Some persons were also given anthrax vaccine or were switched to alternative antibacterial drugs. No one who received CIPRO or other therapies as prophylactic treatment subsequently developed inhalational anthrax. The number of persons who received CIPRO as all or part of their post-exposure prophylaxis regimen is unknown.


A placebo-controlled animal study in African green monkeys exposed to an inhaled mean dose of 110 LD50 (range 92 to 127 LD50) of Yersinia pestis (CO92 strain) was conducted. The minimal inhibitory concentration (MIC) of ciprofloxacin for the Y. pestis strain used in this study was 0.015 mcg/mL. Mean peak serum concentrations of ciprofloxacin achieved at the end of a single 60 minute infusion were 3.49 ± mcg/mL 0.55 mcg/mL, 3.91 mcg/mL ± 0.58 mcg/mL and 4.03 mcg/mL ± 1.22 mcg/mL on Day 2, Day 6 and Day 10 of treatment in African green monkeys, respectively All trough concentrations (Day 2, Day 6 and Day 10) were < 0.5 mcg /mL. Animals were randomized to receive either a 10-day regimen of intravenous ciprofloxacin 15 mg/kg, or placebo beginning when animals were found to be febrile (a body temperature greater than 1.5oC over baseline for two hours), or at 76 hours post-challenge, whichever occurred sooner. Mortality in the ciprofloxacin group was significantly lower (1/10) compared to the placebo group (2/2) [difference: -90.0%, 95% exact confidence interval: -99.8% to -5.8%]. The one ciprofloxacin-treated animal that died did not receive the proposed dose of ciprofloxacin due to a failure of the administration catheter. Circulating ciprofloxacin concentration was below 0.5 mcg/mL at all timepoints tested in this animal. It became culture negative on Day 2 of treatment, but had a resurgence of low grade bacteremia on Day 6 after treatment initiation. Terminal blood culture in this animal was negative.12


1. 21 CFR 314.510 (Subpart H–Accelerated Approval of New Drugs for Life-Threatening Illnesses).

5. Clinical and Laboratory Standards Institute (CLSI). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard–9th Edition. CLSI Document M7-A9 [2012]. Clinical and Laboratory Standards Institute, 950 West Valley Rd., Suite 2500, Wayne, PA. 19087-1898.

6. Clinical and Laboratory Standards Institute (CLSI). Performance Standards for Antimicrobial Susceptibility Testing; 24th Informational Supplement. CLSI Document M100 S24 [2014]. Clinical and Laboratory Standards Institute, 950 West Valley Rd., Suite 2500, Wayne, PA. 19087-1898.

7. Clinical and Laboratory Standards Institute (CLSI). Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria; Approved Guideline–2nd Edition. CLSI Document M45-A2 [2010]. Clinical and Laboratory Standards Institute, 950 West Valley Rd., Suite 2500, Wayne, PA. 19087-1898.

8. Clinical and Laboratory Standards Institute (CLSI), Performance Standards for Antimicrobial Disk Susceptibility Tests; Approved Standard–11th Edition. CLSI Document M2-A11[2012]. Clinical and Laboratory Standards Institute, 950 West Valley Rd., Suite 2500, Wayne, PA. 19087-1898.

9. CReport presented at the FDA's Anti-Infective Drug and Dermatological Drug Product's Advisory Committee meeting, March 31, 1993, Silver Spring, MD. Report available from FDA, CDER, Advisors and Consultants Staff, HFD-21, 1901 Chapman Avenue, Room 200, Rockville, MD 20852, USA.

10. Kelly DJ, et al. Serum concentrations of penicillin, doxycycline, and ciprofloxacin during prolonged therapy in rhesus monkeys. J Infect Dis 1992; 166:1184-7.

11. Friedlander AM, et al. Postexposure prophylaxis against experimental inhalational anthrax. J Infect Dis 1993; 167:1239-42.

12. Anti-infective Drugs Advisory Committee Meeting, April 3, 2012 -The efficacy of Ciprofloxacin for treatment of Pneumonic Plague.

Last reviewed on RxList: 1/31/2017
This monograph has been modified to include the generic and brand name in many instances.

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