FDA-approved
Immune & defense
Polymyxin B / Colistin
Also called polymyxin E (colistimethate)
Polymyxin B and colistin are FDA-approved last-resort antibiotics for resistant Gram-negative infections. Learn how they work and why their use is limited.
- Reviewed by
- William Maish, MD MBA MPH, Clinical Product Lead
- Published
- October 5, 2026
- Last updated
- October 5, 2026
Key takeaway
Polymyxin B and colistin (colistin is also called polymyxin E) are older cyclic lipopeptide antibiotics that have returned to use as last-resort treatments for multidrug-resistant Gram-negative infections. They punch holes in the bacterial outer membrane but carry meaningful kidney and nerve toxicity. Both have long been FDA-approved in the U.S. and are reserved for serious infections with few other options.
At a glance
| What they are | Polymyxin cyclic lipopeptide antibiotics active against Gram-negative bacteria |
| Approved for | Serious infections caused by susceptible Gram-negative organisms, including multidrug-resistant strains |
| Key fact | In a randomized trial, adding a carbapenem to colistin did not improve outcomes over colistin alone for carbapenem-resistant infections |
| Route | Intravenous or intramuscular (polymyxin B is also used topically and in eye preparations) |
| Evidence strength | FDA approved |
What are polymyxin B and colistin?
Polymyxin B and colistin are polymyxins, a class of cyclic lipopeptide antibiotics discovered in the 1940s. They fell out of favor decades ago because of toxicity, but the rise of highly resistant Gram-negative bacteria such as carbapenem-resistant Acinetobacter, Pseudomonas, and Klebsiella has brought them back as drugs of last resort. Colistin is also known as polymyxin E, and it is given as an inactive prodrug, colistimethate, that converts to active colistin in the body. Both have been FDA-approved for many years.
How do they work?
Polymyxins act like a detergent on the Gram-negative outer membrane. Their positively charged ring binds the lipid A portion of lipopolysaccharide, the molecule that forms the outer surface of Gram-negative bacteria, and displaces the stabilizing magnesium and calcium ions. This disrupts and permeabilizes both the outer and inner membranes, so the cell's contents leak out and it dies. Polymyxins also bind and neutralize circulating lipopolysaccharide (endotoxin).
Because their target is unique to Gram-negative bacteria, polymyxins are used specifically against those organisms, often when nothing else is active. Their use is limited by toxicity, and the best way to use them is still debated: in the AIDA trial, adding a carbapenem to colistin did not improve1 outcomes over colistin alone for serious carbapenem-resistant Gram-negative infections. Dosing is a careful balance between efficacy and kidney injury.
Is it FDA-approved?
Yes. Both polymyxin B and colistin (as colistimethate) were approved2 by the FDA decades ago for serious Gram-negative infections. The table below compares how their labels warn about toxicity.
| Product | Label warning |
| Injectable polymyxin B | Boxed warning about kidney toxicity and neurotoxicity, including the risk of respiratory paralysis from neuromuscular blockade |
| Colistimethate | Warnings about the same risks, though not in boxed form |
What are they prescribed for?
Polymyxin B and colistin are prescribed for serious infections caused by susceptible Gram-negative bacteria, particularly multidrug-resistant organisms for which few other antibiotics remain effective. Polymyxin B is also widely used topically and in eye preparations, often in combination products. They are generally reserved for situations where safer agents cannot be used.
What are the side effects?
Common and dose-related effects include:
- Kidney injury, the main dose-limiting toxicity
- Numbness, tingling, and dizziness (neurotoxicity)
More serious considerations include:
- Acute kidney injury, which can be significant
- Neuromuscular blockade that can impair breathing, especially with other such drugs
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Sources: FDA prescribing information for polymyxin B and colistimethate (Coly-Mycin); the AIDA trial (Paul et al., Lancet Infectious Diseases, 2018, DOI: 10.1016/S1473-3099(18)30099-91), indexed on PubMed. This document is educational and does not constitute medical advice.