What it is
Polymyxin B is a ring-shaped peptide with a fatty-acid tail, made by a soil bacterium. The injection label calls it "one of a group of basic polypeptide antibiotics derived from B polymyxa." What is sold is not one molecule but a mixture — as is the anticancer antibiotic bleomycin: polymyxin B sulfate is "the sulfate salt of Polymyxins B1 and B2" (label). The main component, polymyxin B1, has the formula C56H98N16O13 (mass 1,203.5) and a 6-methyloctanoyl fatty acid at the start of its chain (ChEBI:8309).
ChEBI describes the whole family as antibiotics with "a cyclic peptide with a long hydrophobic tail." That design — a positively charged peptide ring plus a greasy tail — is shared with daptomycin, which targets the opposite kind of bacteria.
How it works
Polymyxins "disrupt the structure of the bacterial cell membrane by interacting with its phospholipids" and are "selectively toxic for Gram-negative bacteria" (ChEBI). Gram-negative bacteria wrap their thin cell wall in an outer membrane, and that membrane is the target; the label's word "basic" means the peptide carries positive charges. Because polymyxin B does not cover Gram-positive germs, combination eye products pair it with bacitracin, whose label describes activity against most Gram-positive bacteria.
Units, not milligrams
Polymyxin B is dosed by potency. The label converts: 1 mg of pure polymyxin B base = 10,000 units, and each injection vial contains 500,000 units. The 2019 international consensus guidelines on the polymyxins name "several different conventions used to describe doses" among the reasons for "significant confusion" in their use (Tsuji et al., Pharmacotherapy 2019, PMID 30710469). Doses on the label are adjusted for kidney function and are decided by the treating team.
Mostly an eye and ear medicine
Our count of the 27 polymyxin B applications in Drugs@FDA (openFDA, 2026-10-14):
| Product type | Listings | Typical partners |
|---|---|---|
| Eye drops and ointments (combinations) | 13 | trimethoprim; neomycin + bacitracin or gramicidin; neomycin + dexamethasone or hydrocortisone |
| Ear drops (combinations) | 7 | neomycin + hydrocortisone |
| Irrigation solutions (combinations) | 2 | neomycin |
| Single-ingredient injection | 5 | — (4 prescription, the oldest, from 1964, discontinued) |
So the molecule most people meet is in an eye or ear drop, not a vein. The oldest application listed is Maxitrol eye drops, NDA 050023, dated 1963-06-06.
The injection and its boxed warning
The injection label names Pseudomonas aeruginosa infections of the urinary tract, meninges and bloodstream, and, "when less potentially toxic drugs are ineffective or contraindicated", certain infections with H. influenzae, E. coli, Aerobacter aerogenes and Klebsiella pneumoniae. For meningitis it "should be administered only by the intrathecal route" — into the spinal fluid.
The boxed warning, summarised as the label states it:
- Hospital only when given into muscle or the spinal fluid, under constant supervision.
- Kidney toxicity: albuminuria, cellular casts and azotaemia; "diminishing urine output and a rising BUN are indications for discontinuing therapy." Dosage is reduced in kidney damage.
- Nerve toxicity: irritability, weakness, drowsiness, numbness, blurred vision, and respiratory paralysis from neuromuscular blockade, especially soon after anaesthesia or muscle relaxants.
- Avoid combining with other kidney- or nerve-toxic drugs, naming bacitracin, streptomycin, neomycin, kanamycin, gentamicin, tobramycin, amikacin and colistin.
- Pregnancy: "safety … has not been established."
The label is an older-format label with no randomised-trial section. The guideline authors make the same point about the whole class: the polymyxins "became available in the 1950s and thus did not undergo contemporary drug development procedures," and their recent return is "as salvage therapy for otherwise untreatable gram-negative infections."
Where it stands
- Approved in the US since the 1960s; 27 applications in Drugs@FDA, 22 of them combination eye, ear or irrigation products (openFDA, 2026-10-14).
- Guidelines: the 2019 international consensus guidelines (ACCP, IDSA, ISAP, SCCM, SIDP; endorsed by ESCMID, conclusions endorsed by EUCAST) were described by their authors as the first consensus recommendations for colistin and polymyxin B.
- Research record: PubMed indexes 8,142 records for "polymyxin B", 167 tagged as randomised trials (2026-10-14).
Related reading
Polymyxin B's most common partner in eye ointments is bacitracin. Two other bacterial peptide antibiotics in the library are daptomycin, a lipopeptide for Gram-positive infections, and the hospital glycopeptide vancomycin, which blocks cell-wall building. An approved drug that is a random mixture of four-amino-acid chains rather than a few defined components is glatiramer acetate (Copaxone).