Does Polymyxin B Contain Penicillin?
No, polymyxin B does not contain penicillin. This is explicitly stated in FDA-approved labeling and multiple clinical guidelines 1, 2, 3.
Direct Evidence from Drug Labeling
The FDA-approved prescribing information for polymyxin B for injection clearly states that it is derived from Bacillus polymyxa and is a polypeptide antibiotic 3. The vaccinia vaccine guidelines from the CDC's Advisory Committee on Immunization Practices explicitly confirm: "Vaccinia vaccine does not contain penicillin" when discussing the trace amounts of polymyxin B sulfate present in that vaccine formulation 1, 2.
Chemical Structure and Class
Polymyxin B is a polypeptide antibiotic, not a beta-lactam antibiotic like penicillin 3. These are completely different drug classes with distinct:
- Chemical structures (polypeptide vs. beta-lactam ring)
- Mechanisms of action (membrane disruption vs. cell wall synthesis inhibition)
- Bacterial targets (gram-negative outer membrane vs. peptidoglycan synthesis)
Clinical Implications for Penicillin-Allergic Patients
Patients with penicillin allergies can safely receive polymyxin B, as there is no cross-reactivity between these drug classes 1, 2. The allergy guidelines specifically note that polymyxin B is among the antibiotics that may be present as trace contaminants in some vaccine formulations, but it is listed separately from penicillin precisely because they are unrelated compounds 1.
Important Caveat
While polymyxin B does not contain penicillin, some research has documented that these antibiotics can be used in combination therapy for certain infections 4, 5, 6, 7. However, this represents concurrent administration of two separate drugs, not the presence of penicillin within polymyxin B itself.
Bottom line: Polymyxin B is penicillin-free and safe for patients with penicillin allergies from a cross-reactivity standpoint.