At a glance
| Members covered | Polymyxin B and colistin / polymyxin E |
|---|---|
| Colistin precursor | Colistimethate sodium |
| Target organisms | Selected susceptible Gram-negative bacteria |
| Major toxicity | Kidney injury and neurotoxicity |
| Product caution | Systemic and topical products differ |
Overview
This grouped entry covers both polymyxin B and colistin while retaining the differences that matter clinically. Polymyxin B is administered as an active antibiotic; many systemic colistin products contain colistimethate sodium, which is converted into active colistin. Formulation, conversion, exposure, and dose-expression conventions differ. These older antibiotics regained importance as resistance narrowed treatment options, but their toxicity makes them specialist tools rather than routine broad-spectrum wellness antimicrobials. Topical combination products containing polymyxin B also exist and must not be confused with systemic injection products. Selection depends on the actual pathogen, susceptibility, infection site, kidney function, and available alternatives.
How it works
Polymyxins interact with lipid components of Gram-negative bacterial outer membranes and disrupt membrane integrity, producing bactericidal activity. Their cationic peptide and lipid portions are both important to this interaction. Colistimethate must first convert to active colistin, whereas polymyxin B is supplied in active form. This chemical difference influences how exposure develops and why a treatment plan cannot be transferred by simply substituting one product name for the other.
What each member contributes
Polymyxin B injection and colistimethate injection have labels for susceptible Gram-negative infections. Susceptibility is not universal across Gram-negative species, and resistance testing matters. Their use against difficult resistant pathogens is an infection-specific decision; evidence of activity against one organism cannot establish effectiveness against an unrelated infection or at a poorly reached site.
Why formulation is part of the evidence
Coly-Mycin M contains colistimethate and expresses potency in terms of colistin base activity, while other jurisdictions or products may use different conventions. Polymyxin B has its own labeling. Studies, exposure targets, and safety observations must be attached to the actual formulation; the class relationship does not make preparation or unit conventions equivalent.
Kidney, nerve, and respiratory risks
Both medicines carry substantial renal and neurologic safety concerns. Neuromuscular blockade can cause respiratory weakness or paralysis, especially with interacting medicines or vulnerable patients. Colistimethate labeling also describes renal tubular electrolyte disturbances. Monitoring and review of other nephrotoxic or neurotoxic exposures are central to systemic treatment.
Is polymyxin in an ointment the same treatment?
No. A skin, ear, or eye combination product delivers different exposure and has different indications from systemic therapy. Conversely, an injectable product is not a substitute for a topical formulation. The shared ingredient name does not establish route interchangeability, adequate tissue exposure, or comparable risks.
Reading this profile
Jurisdiction: United States; indications and availability vary by formulation and country.. Molecular class: Related cyclic lipopeptide antibiotics; colistimethate is a colistin prodrug.
This profile was written with AI-assisted research and checked against the linked sources. It has not been represented as a clinician’s review. Evidence summaries describe the cited studies and products; they do not establish that a separately purchased preparation is equivalent.
Sources & references
Follow the original material. Product labels define specific formulations and uses; trial registration describes a study and does not by itself establish benefit.
- Polymyxin B injection — US prescribing information ↗Regulatory source · Checked 2026-10-27
- Coly-Mycin M — US prescribing information ↗Regulatory source · Checked 2026-10-27