Trimethoprim-Polymyxin Does NOT Contain a Sulfa Component
Trimethoprim-polymyxin ophthalmic solution contains only trimethoprim and polymyxin B, without any sulfonamide (sulfa) component, making it safe for patients with sulfa allergies. 1
Composition Breakdown
The FDA-approved formulation of polymyxin B sulfate and trimethoprim ophthalmic solution contains 1:
- Trimethoprim 1 mg/mL (as trimethoprim sulfate salt)
- Polymyxin B sulfate 10,000 units/mL
- Preservative: benzalkonium chloride 0.04 mg/mL
- Inactive ingredients: water, sodium chloride, sulfuric acid
Critical Distinction: Trimethoprim vs. Trimethoprim-Sulfamethoxazole
The confusion arises because trimethoprim is commonly combined with sulfamethoxazole (a sulfonamide) in oral formulations like TMP-SMX (Bactrim, Septra), but this is an entirely different medication. 2
Key Differences:
Trimethoprim-sulfamethoxazole (TMP-SMX): Contains the sulfonamide sulfamethoxazole and carries all sulfa-related risks including Stevens-Johnson syndrome, hemolytic anemia in G6PD deficiency, and hypersensitivity reactions 2
Trimethoprim-polymyxin: Contains NO sulfonamide component; polymyxin B is a polypeptide antibiotic, not a sulfa drug 1, 3
Clinical Evidence Supporting Safety
Research demonstrates that trimethoprim-polymyxin combinations are effective without requiring a sulfonamide component 3:
- Clinical trials comparing trimethoprim-sulfacetamide-polymyxin B versus trimethoprim-polymyxin B alone showed no loss of effectiveness when the sulfonamide (sulfacetamide) was removed 3
- The trimethoprim-polymyxin combination demonstrated synergistic or additive antibacterial activity against gram-negative bacilli in 85% of cases without any sulfa component 4
Practical Implications for Sulfa-Allergic Patients
Patients with documented sulfa allergies can safely receive trimethoprim-polymyxin ophthalmic solution, as it contains no sulfonamide component. 1, 3
Common Pitfall to Avoid:
Do not confuse the "sulfate" salt form (trimethoprim sulfate, polymyxin B sulfate) with "sulfonamide" antibiotics. Sulfate salts are simply formulation vehicles and do NOT cause sulfa allergies. The allergenic component in sulfa drugs is the sulfonamide functional group (SO₂NH₂), which is absent in trimethoprim-polymyxin formulations 1, 5.