Colistin (colistimethate) !

A last-resort polymyxin for multidrug-resistant Pseudomonas, Acinetobacter and Klebsiella; it can injure the kidney and nerves and stop breathing.

Updated 2026-11-01

Class: Polymyxins
Target: Lipid A of the lipopolysaccharide, gram-negative outer membrane
ATC code: J01XB01
WHO Model List of Essential Medicines

Also called: polymyxin E, colistimethate sodium

Mechanism of action

Colistin is a cationic peptide with a fatty tail. It binds the negatively charged lipid A of lipopolysaccharide, inserts into the outer and then the inner membrane, and disrupts them like a detergent; the bacterium leaks and dies. It is given as colistimethate, an inactive prodrug that is slowly converted to colistin in the blood.

Colistimethate is converted to active colistin
Binds lipid A of the outer membrane
Disrupts the membranes like a detergent
The gram-negative bacterium leaks and dies

Pharmacokinetics

Absorption: Not absorbed by mouth; given IV, IM or by inhalation.
Metabolism: The prodrug colistimethate is hydrolysed slowly to active colistin; levels vary a lot between patients.
Elimination: The prodrug is cleared mainly by the kidney (which suits urinary infections); active colistin mostly by other routes.
Half-life: Serum levels fall with a half-life of about 2 to 3 hours after IV or IM colistimethate.

Indications

Serious infections by multidrug-resistant gram-negative bacilli (Pseudomonas aeruginosa, Acinetobacter, Klebsiella, Enterobacter, E. coli) when safer options fail. MRSA and other multidrug-resistant organism infections →
Urinary tract infections by multidrug-resistant gram-negatives (colistimethate reaches the urine well). Urinary tract infection (acute cystitis) →
Inhaled, as an adjunct in hospital-acquired pneumonia and in cystic fibrosis. Cystic fibrosis →

Adverse effects

Common

Tingling around the mouth and in the limbs, numbness, itching
Dizziness, vertigo, slurred speech
Gastrointestinal upset, fever, urticaria and rash

Contraindications

Hypersensitivity to colistimethate or polymyxins

Key interactions

Aminoglycosides and polymyxin B: Added neuromuscular blockade and nephrotoxicity; concurrent use is avoided or done with extreme caution.
Curare-like muscle relaxants, succinylcholine, anaesthetic agents, sodium citrate: Potentiate the blockade: risk of respiratory paralysis.
Vancomycin and other nephrotoxic drugs: More kidney injury.

What is monitored

Creatinine and urine output at baseline and often during treatment
Respiratory rate and muscle strength, especially after surgery
Neurological symptoms (paresthesia, confusion, vertigo)
Electrolytes

Administration and safety

Strength may be labelled as colistin base activity or in other units depending on the country and product: the unit is confirmed with pharmacy to avoid dosing errors.
Reconstituted and given by slow IV injection, IV infusion or IM; the inhaled preparation is used only by nebulizer.

Nursing care

Before starting: kidney function, weight, other nephrotoxic drugs and planned anaesthesia.
Record intake and output; report falling urine output or rising creatinine.
Watch breathing and muscle weakness, especially after surgery or with relaxants.
Teaching: tingling or dizziness may occur; avoid driving or using machinery while on it.

Pregnancy and breastfeeding

Animal studies showed malformations at doses below the human dose, and human data are lacking; it is used in pregnancy only when no safer option exists.

Test yourself

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

Why is colistin called a last-resort antibiotic?

It still works against many multidrug-resistant gram-negatives, but its kidney and nerve toxicity is high, so it is kept for when safer drugs fail.

After surgery, a patient on colistimethate has shallow breathing and weakness. What has likely happened?

Neuromuscular blockade by colistin, potentiated by anaesthetics and muscle relaxants, which can lead to apnea.

Why is colistimethate preferred over polymyxin B for urinary infections?

The prodrug colistimethate is cleared by the kidney and reaches the urine, while polymyxin B is cleared mostly by non-renal routes.

Which organisms resist colistin naturally?

Proteus and Serratia (and Burkholderia), plus all gram-positive bacteria.

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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 - COLISTIMETHATE- colistimethate sodium injection, powder, for solution (DailyMed (U.S. National Library of Medicine), 2026); Colistimethate Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 59 (McGraw Hill, 2023).

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Representative model, for educational purposes only.