Polymyxin B sulfate is an antibiotic that kills Gram-negative bacteria by destroying their outer cell membranes. It exists in two very different worlds: as an ingredient in over-the-counter triple antibiotic ointments you can buy at any pharmacy, and as an injectable “last resort” drug reserved for serious, drug-resistant infections in hospital settings. Which version matters to you depends entirely on why you’re looking it up.
How Polymyxin B Kills Bacteria
Gram-negative bacteria have a protective outer membrane built partly from a molecule called lipopolysaccharide, or LPS. Polymyxin B carries multiple positive electrical charges that are strongly attracted to the negatively charged phosphate groups on the lipid A portion of LPS. Once the drug latches onto these sites, its fatty acid tail (optimally seven to nine carbon atoms long) inserts into the membrane like a wedge, punching holes in the cell’s outer wall.
This initial breach destabilizes the outer membrane, allowing polymyxin B to reach the inner membrane and disrupt it as well. The result is that the bacterial cell essentially leaks to death. Because the drug physically tears apart the membrane rather than interfering with a single enzyme or protein, it works quickly and is effective against bacteria that have developed resistance to most other antibiotics.
Topical Uses You’ll Find Over the Counter
The form of polymyxin B sulfate most people encounter is in combination ointments designed to prevent infection in minor cuts, scrapes, and burns. The best-known example is Neosporin, which combines polymyxin B with neomycin and bacitracin. Generic versions are sold simply as “triple antibiotic ointment.” These products are available without a prescription and are applied directly to the skin.
Polymyxin B sulfate also appears in prescription eye drops, typically paired with trimethoprim, for treating bacterial eye infections like conjunctivitis. In these topical and ophthalmic forms, the drug stays at the surface of the body and very little is absorbed into the bloodstream, which keeps systemic side effects minimal.
Intravenous Use for Serious Infections
When given by injection, polymyxin B sulfate targets life-threatening infections caused by multidrug-resistant Gram-negative bacteria, particularly Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. These are pathogens that commonly cause ventilator-associated pneumonia, bloodstream infections, and other hospital-acquired conditions where standard antibiotics have failed.
Current guidance from the Infectious Diseases Society of America positions polymyxin B as an option primarily when newer, preferred antibiotics aren’t effective. For difficult-to-treat P. aeruginosa infections, for instance, polymyxin B may be considered in combination with another antibiotic when first-line drugs test resistant. The IDSA specifically recommends against using polymyxin B for urinary tract infections, because the drug is cleared through non-kidney pathways and doesn’t concentrate well in urine.
When polymyxin B is compared to colistin (a closely related polymyxin), clinicians generally prefer polymyxin B for infections outside the urinary tract. Unlike colistin, which is given as an inactive prodrug that the body must convert, polymyxin B reaches predictable blood levels more reliably. It also carries a potentially lower risk of kidney damage, though head-to-head comparisons remain limited.
Side Effects of Systemic Treatment
The two major concerns with intravenous polymyxin B are kidney damage and nerve-related toxicity. In a study of 60 intensive care patients with no prior kidney problems, 35% developed some degree of kidney injury after receiving the drug. Most of those cases were mild, but about 14% of the affected patients progressed to severe kidney injury. Across the broader literature, the reported rate of kidney damage ranges widely, from 4% to 60%, depending on the patient population and how kidney injury is defined.
Neurotoxicity can show up as numbness or tingling in the face and extremities, dizziness, or in rare cases, respiratory muscle weakness. The drug’s half-life in the blood is roughly 11.5 hours, and clearance happens mostly through non-kidney routes. This means that unlike many antibiotics, dosing doesn’t change dramatically for patients with reduced kidney function, a somewhat unusual pharmacological trait that has practical implications for ICU dosing.
Drug Interactions to Be Aware Of
Polymyxin B can amplify the effects of neuromuscular blocking agents, the drugs used during surgery to keep muscles relaxed. If both are given together, there’s a heightened risk of prolonged muscle paralysis. It also interacts with other medications that stress the kidneys, including certain immunosuppressants and some blood pressure drugs, potentially compounding the risk of kidney injury. For the ophthalmic form, interactions with certain heart rhythm medications have been flagged, though the clinical significance with eye drops is much lower than with IV administration.
How Bacteria Develop Resistance
Because polymyxin B works by binding to the negatively charged phosphate groups on lipid A, bacteria resist it by chemically modifying those binding sites. Some species, like Proteus mirabilis, are naturally resistant because they constantly produce enzymes that attach positively charged sugar or chemical groups to lipid A, reducing the negative charge that polymyxin B needs to grab onto. This type of intrinsic resistance doesn’t spread between bacterial species.
The bigger concern is acquired resistance through a gene called mcr-1, first identified in bacteria from a pig farm in China in 2015. This gene sits on a plasmid, a small piece of DNA that bacteria can pass to one another like sharing a flash drive. The enzyme it produces transfers a chemical group called phosphoethanolamine onto lipid A, neutralizing the charge that polymyxin B targets. Ten variants of the mcr gene family have been identified so far. The widespread use of colistin in livestock farming has accelerated the spread of these resistance genes, raising concerns about losing one of medicine’s few remaining options against the most resistant infections.
Units and Dosing Basics
Polymyxin B sulfate is measured in international units rather than milligrams, which can be confusing. One international unit equals the activity contained in 0.000127 mg of the international standard. In practice, this means a typical dose measured in tens of thousands of units translates to a relatively small weight of actual drug. This unit-based system exists because the antibiotic’s potency can vary between manufacturing batches, and measuring biological activity provides a more consistent gauge of what patients actually receive.