Polymyxin B, sold under the brand name Poly-Rx among others, is an antibiotic used to treat meningitis, pneumonia, sepsis, and urinary tract infections.

[7][8] Polymyxin B has been used to treat urinary tract infections and meningitis caused by Pseudomonas aeruginosa and Haemophilus influenzae, respectively.

An effective use of polymyxin B is found in patients with refractory septic shock, that is, without positive outcome to the administration of standard treatments (increase in volemia and other antibiotics).

The obstacle of the toxicity of polymyxin B is bypassed by extracorporeal circulation with perfusion of venous blood through a cartridge on whose fibers polymyxin B is covalently fixed; in this way the antibiotic exerts its bactericidal function but is not released into the blood since it remains fully attached to the fiber.

However, it still detectably increases the permeability of the bacterial cell wall to other antibiotics, indicating that it still causes some degree of membrane disorganization.

Polymyxin envelope stress assays such as this have been used for the study of small RNA (sRNA) responses in Salmonella enterica.