Mechanism of Action

Fluoroquinolones inhibit bacterial DNA gyrase and topoisomerase IV — the enzymes bacteria need to uncoil and replicate DNA. Without them the chromosome fragments and the organism dies, making these drugs bactericidal. They cover a broad spectrum: gram-negatives (E. coli, and Pseudomonas for ciprofloxacin), atypicals (Mycoplasma, Legionella), and respiratory gram-positives (S. pneumoniae for levofloxacin/moxifloxacin — the 'respiratory fluoroquinolones'). Because of serious class toxicities the FDA reserves them for infections without safer alternatives. Oral bioavailability nearly equals IV, so oral-to-IV conversion is common.

Common Medications

ciprofloxacinPrototype

strongest gram-negative + Pseudomonas coverage

levofloxacin

respiratory fluoroquinolone; gram-positive/S. pneumoniae

moxifloxacin

respiratory fluoroquinolone; gram-positive coverage

Indications

complicated urinary tract infection

NOT first-line for uncomplicated UTI

hospital-acquired pneumonia

anthrax prophylaxis

bone and joint infections

certain cases

reserved for no safer alternative

FDA restriction — not first-line for uncomplicated UTI, sinusitis, or bronchitis

Side Effects

photosensitivity

teach sunscreen and protective clothing

dizziness

mild CNS effect

headache

mild CNS effect

nausea

Contraindications & Interactions

Contraindications

pregnancy

generally avoid — risk of cartilage damage

age under 18

generally avoid — cartilage/joint damage

known aortic aneurysm

boxed-warning aortic rupture risk; choose a safer agent

Interactions

calcium

divalent cation chelates the drug — separate 2 h before / 6 h after; includes antacids and dairy

magnesium

antacids — chelation; separate dosing

iron

chelation — separate dosing

aluminum antacids

chelation — separate dosing

zinc

multivitamins — chelation; separate dosing

other QT-prolonging drugs

additive QT prolongation — risk of fatal dysrhythmia

concurrent corticosteroids

amplifies tendon rupture risk

Administration & Monitoring

assess for tendon pain each visit

ask about heel/Achilles pain or swelling every visit

hold drug for tendon painHold

stop the fluoroquinolone and notify provider immediately — do not wait

separate from cation products

give 2 h before or 6 h after calcium/magnesium/iron/aluminum/zinc, antacids, dairy, multivitamins

monitor ECG / QT interval

especially with other QT-prolonging drugs or electrolyte abnormalities

question order when safer alternative exists

advocate against fluoroquinolone for uncomplicated UTI, sinusitis, bronchitis, or known AAA

watch for C. difficile diarrhea

report severe or persistent watery diarrhea

Patient Teaching

stop drug and report tendon pain immediately

heel/Achilles pain or swelling — black-box tendon rupture

report extremity numbness or burning

peripheral neuropathy (pain/burning/tingling/numbness) — may be permanent

report confusion or seizures

CNS black-box effect

separate from antacids, dairy, iron, multivitamins

2 h before or 6 h after — cation chelation kills the dose

use sunscreen and protective clothing

photosensitivity

report new diarrhea

C. difficile risk

stay hydrated

Report Nowescalate immediately

tendon rupture HallmarkBlack Box

FDA boxed warning; tendinitis/rupture (most often Achilles), during therapy or weeks after; risk multiplied by concurrent corticosteroids and age >60 — stop drug and report tendon pain/swelling immediately

peripheral neuropathy

FDA boxed warning; pain/burning/tingling/numbness in extremities — may be irreversible even after discontinuation

CNS effects

FDA boxed warning; seizures, confusion, toxic psychosis, raised ICP — higher risk with seizure history or renal impairment

aortic aneurysm rupture

FDA boxed warning; impaired collagen cross-linking — avoid with known aneurysm; advocate a safer antibiotic

QT prolongation Hallmarkmonitor ECG

NOT an FDA boxed warning, but report-now — risk of torsades; avoid combining with other QT-prolonging drugs and correct electrolytes

Clinical Pearl

Tendons, nerves, brain — fluoroquinolones attack connective and neural tissue, the heart of their FDA black-box warnings. If a client on cipro says 'my heel hurts,' stop the drug first and ask questions later. And anything with a metal cation — antacids, dairy, iron, multivitamins — eats the dose: separate by 2 hours before or 6 hours after.