Ciprofloxacin !

The fluoroquinolone most active against gram-negatives such as Pseudomonas. Used for kidney, prostate and gut infections, with tendon and nerve warnings.

Updated 2026-11-01

Class: Fluoroquinolones
Enzyme: Bacterial DNA gyrase and topoisomerase IV
ATC code: J01MA02
WHO Model List of Essential Medicines

Bacterial DNA gyrase and topoisomerase IVCiprofloxacin

The figure shows ciprofloxacin wedged into DNA cut by Staphylococcus aureus DNA gyrase.

Target in 3D: Bacterial DNA gyrase and topoisomerase IV. PDB structure 2XCT (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Ciprofloxacin binds DNA gyrase and topoisomerase IV at the moment they have cut both DNA strands. The enzyme cannot rejoin the DNA, so the cut becomes a lasting double-strand break and the bacterium dies. Its strongest activity is against gram-negative bacilli, including Pseudomonas aeruginosa; it is weaker against pneumococcus.

Enters the bacterium through porins
Binds gyrase or topoisomerase IV on cut DNA
The DNA cannot be rejoined
Double-strand breaks kill the cell

Pharmacokinetics

Absorption: Well absorbed by mouth; antacids with magnesium or aluminium can cut absorption by up to 90%. Also given IV.
Distribution: Wide; plasma protein binding 20–40%.
Elimination: Kidney and other routes; the dose is adjusted in kidney failure.
Half-life: About 4 hours with normal kidney function.

Indications

Acute pyelonephritis (WHO first choice in mild to moderate disease) Acute pyelonephritis →
Bacterial prostatitis Prostatitis →
Complicated urinary tract infection when other options are unsuitable Urinary tract infection (acute cystitis) →
Typhoid and paratyphoid fever and infectious diarrhoea (WHO list)
Inhalational anthrax after exposure and plague; ear drops for external otitis

Adverse effects

Common

Nausea, diarrhoea and abdominal pain
Headache, dizziness and insomnia
Rash and photosensitivity

Contraindications

Allergy to ciprofloxacin or any quinolone
Use together with tizanidine

Key interactions

Tizanidine: Contraindicated: ciprofloxacin blocks its breakdown by CYP1A2, causing low blood pressure and sedation.
Theophylline and caffeine: Levels rise (CYP1A2 inhibition): risk of seizures and arrhythmia with theophylline.
Antacids, calcium, iron, zinc, dairy products alone: Bind ciprofloxacin in the gut; the doses are spaced.
Warfarin and antidiabetic drugs: Stronger anticoagulant effect; risk of hypoglycaemia.

What is monitored

Tendon pain, numbness or tingling, mood or sleep changes
Blood glucose in diabetes
ECG in patients at risk of QT prolongation; kidney function

Administration and safety

Oral doses are separated from antacids, minerals and dairy-only drinks
Plenty of fluid helps avoid crystals in the urine
Infuse the IV form slowly into a large vein

Nursing care

Ask about previous tendon problems, myasthenia gravis and corticosteroid use
Teach to stop and report tendon pain, numbness, confusion or palpitations
Advise sun protection
Check glucose more often in diabetes

Pregnancy and breastfeeding

Animal studies have not shown malformations and human data have not shown a clear increase in birth defects, but safer options are usually chosen.

Test yourself

Think of the answer, then tap the question to see it.

Which bacteria are the main strength of ciprofloxacin?

Gram-negative bacilli, including Pseudomonas aeruginosa.

Why is tizanidine contraindicated with ciprofloxacin?

Ciprofloxacin inhibits CYP1A2, the enzyme that clears tizanidine, causing severe hypotension and sedation.

Which adverse effects form the boxed warning?

Tendinitis and tendon rupture, peripheral neuropathy, central nervous system effects and worsening of myasthenia gravis.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - CIPROFLOXACIN tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026); Ciprofloxacin: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025); The twinned 3.35A structure of S. aureus Gyrase complex with Ciprofloxacin and DNA (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.