Using Polymyxin B If You Are Allergic To Neomycin

In our latest question and answer, the pharmacist discusses the safety of using polymyxin B eye drops if you are allergic to neomycin.

Dec 18, 2017

Jamie asked

If I am allergic to neomycin, can I use the eye drop polymyxin B sulfate and trimethoprim ophthalmic solution?


Polymyxin B and neomycin are in two distinct medication classes and there is not a known cross-reactivity between them. In other words, being allergic to one should not make you any more susceptible to being allergic to the other. It should be noted however, that some neomycin products are formulated as neomycin sulfate. Therefore, it is important to ensure that you are not allergic to 'sulfa' or 'sulfate' products as most polymyxin products are formulated as a sulfate.

Polymyxin B and trimethoprim is a commonly used ophthalmic medication used for the treatment of bacterial eye infections. The two ingredients works synergistically.

Polymyxin B

Polymyxin B binds to phospholipids in the bacterial cell membrane, destroying it. It is most effective against gram negative bacteria.


Trimethoprim interferes with folate synthesis in a variety of bacteria, both gram positive and gram negative. Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid, disrupting bacterial development.

To reiterate, there is no contraindication for the use of polymyxin B/Trimethoprim ophthalmic products if you have a neomycin allergy. It is important to confirm however, that you do not have a sulfate allergy as both neomycin and polymyxin B can be formulated in sulfate form.

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