Polymyxin B
**Polymyxin B** is a polypeptide antibiotic primarily effective against **Gram-negative** organisms, particularly *Pseudomonas aeruginosa*. Gram-positive bacteria are usually resistant. > **Clinical Warning:** Polymyxins are highly nephrotoxic and are considered too toxic for systemic use. They are restricted to topical, ophthalmic, and otic applications. Because of their strongly basic nature, they are not absorbed from the gastrointestinal tract.
Mechanism: Acts as a **cationic surface-acting agent** → binds to the bacterial cell membrane → causes concentration-dependent disruption of the outer membrane of Gram-negative bacteria → leakage of intracellular contents and cell death.
Dosing by species
- External pseudomonal infections (e.g., keratoconjunctivitis, otitis externa) · Apply topically as directed by specific product formulation · topical · As directed · Until clinical resolution · Do not use if tympanum is ruptured.
- External pseudomonal infections (e.g., keratoconjunctivitis, otitis externa) · Apply topically as directed by specific product formulation · topical · As directed · Until clinical resolution · Do not use if tympanum is ruptured.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Ruptured tympanic membrane (tympanum)
- Systemic administration
Adverse effects
- Nephrotoxicity (if absorbed systemically)
- Ototoxicity (potential, especially if tympanic membrane is ruptured)
- Local irritation
Drug interactions
- Other antibacterial agents · Acts synergistically by disrupting the outer and cytoplasmic membranes, improving penetration of other agents into bacterial cells. · moderate
- Chlorhexidine · Cationic detergents potentiate the antibacterial effects of polymyxin B against Pseudomonas aeruginosa. · moderate
- EDTA · Chelating agents potentiate the antibacterial effects of polymyxin B against Pseudomonas aeruginosa. · moderate
Monitoring
- Resolution of clinical signs (ear, eye, or skin infection)
- Tympanic membrane integrity (must be verified before otic use)
Overdose
Systemic absorption (e.g., from ingestion of large amounts or application to extensive deep wounds) can lead to severe **nephrotoxicity** and **neurotoxicity/ototoxicity**. Treatment is supportive.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.