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Pharmacology · Polymyxins

Polymyxin B !

A polymyxin given IV, IM or intrathecally for resistant gram-negatives, and used topically in eye and ear drops; nephrotoxic and neurotoxic.

Updated 2026-11-01

  • Class: Polymyxins
  • Target: Lipid A of the lipopolysaccharide, gram-negative outer membrane
  • ATC code: J01XB02
  • WHO Model List of Essential Medicines

Mechanism of action

Polymyxin B binds the lipid A of lipopolysaccharide and the phospholipids of the cell membrane and increases its permeability, killing almost all gram-negative bacilli except the Proteus group. Unlike colistimethate, it is given directly in its active form.

  1. Binds lipid A and membrane phospholipids
  2. Membrane permeability rises
  3. The gram-negative bacillus dies

Pharmacokinetics

Absorption
Not absorbed from the gut or intact skin; given IV, IM, intrathecally or topically.
Distribution
Loses much of its activity in the presence of serum, so blood levels of active drug are low; in meningitis it is given only intrathecally.
Elimination
Mainly by non-renal routes.

Indications

Adverse effects

Common

  • Facial flushing, dizziness, drowsiness, circumoral and stocking-glove paresthesia
  • Severe pain at intramuscular sites; thrombophlebitis at IV sites
  • Drug fever, urticarial rash
  • Meningeal irritation after intrathecal use

Contraindications

  • Previous hypersensitivity to polymyxins

Key interactions

Aminoglycosides, colistin, bacitracin, vancomycin and other nephrotoxic or neurotoxic drugs
Added kidney and nerve damage; avoided at the same time or in sequence.
Curare-like relaxants, succinylcholine, ether, sodium citrate
Can precipitate respiratory depression.

What is monitored

  • Kidney function before and during treatment; urinalysis for albumin and casts
  • Neurological signs and breathing
  • Cerebrospinal fluid cultures and glucose during intrathecal treatment

Administration and safety

  • IM and intrathecal use only in hospitalized patients under constant supervision.
  • IM injection is not routine because it is very painful, especially in children.
  • Doses are written in units: check units carefully against the order.
  • Inspect IV sites for thrombophlebitis.

Nursing care

  • Baseline creatinine and urinalysis; daily intake and output.
  • Watch for perioral numbness, drowsiness, weakness or shallow breathing.
  • Extra caution after anaesthesia or with muscle relaxants.
  • Teaching: report numbness, vision change, less urine or trouble breathing.

Pregnancy and breastfeeding

Safety in human pregnancy has not been established; systemic use is reserved for situations without safer alternatives.

Test yourself

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

How is polymyxin B meningitis treatment given, and why?

Only by the intrathecal route, directly into the cerebrospinal fluid; the label reserves meningeal treatment to this route.

Which group of gram-negative bacilli does polymyxin B not kill?

The Proteus group (Serratia is also resistant to polymyxins).

What is a key pharmacological difference between polymyxin B and colistimethate?

Polymyxin B is given in active form and cleared mostly outside the kidney; colistimethate is a prodrug cleared by the kidney.

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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 - POLYMYXIN B- polymyxin b sulfate injection, powder, for solution (DailyMed (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.