Polymyxin B

Polymyxin B is colistin's close cousin — same lipopeptide mechanism, differing by a single amino acid — but genuinely different in clinical practice because it's given as the active drug directly rather than as a prodrug, giving it more predictable pharmacokinetics in patients with kidney impairment.

Nearly identical to colistin, with one practical difference

Polymyxin B is a cyclic lipopeptide antibiotic closely related to colistin — same lipid-A-binding, membrane-disrupting mechanism, differing by a single amino acid substitution. The genuinely useful clinical distinction is pharmacokinetic, not mechanistic: polymyxin B is administered as the active compound itself and is cleared predominantly through non-renal pathways, so its exposure is comparatively predictable regardless of kidney function. Colistin, by contrast, is given as the inactive prodrug colistimethate sodium, which is renally cleared and converts to active drug at a variable rate — a real source of dosing unpredictability that polymyxin B largely avoids.

The comparative toxicity question — genuinely unresolved

Which drug is less nephrotoxic is a real, ongoing debate rather than a settled fact: at least one prospective study found substantially lower nephrotoxicity with polymyxin B than colistin (roughly 12% versus 39% in one comparison), while other analyses have found polymyxin B similarly or even more nephrotoxic depending on the comparator and dosing regimen used. We're stating this plainly as unresolved rather than picking a side neither the evidence nor most clinical guidelines currently settle.

Where you've probably already encountered it

Beyond hospital IV use for resistant infections, polymyxin B (not colistin) is the polymyxin found in common over-the-counter and prescription combination products: Polysporin (bacitracin/polymyxin B), Neosporin (neomycin/polymyxin B/bacitracin), and Polytrim (polymyxin B/trimethoprim) eye drops for bacterial conjunctivitis — genuinely ubiquitous, low-profile uses of the same drug that also serves as a last-resort IV antibiotic.