Can I use Polymyxin B/Trimethoprim drops from 6 months ago that have not expired?

If you used these drops for a previous case of conjuctivitis, you should probably get a new bottle, in case the old bottle got contaminated by your other case of conjunctivitis. Once opened the drops can become colonized with bacteria quite easily, especially if they got exposed to your last bout of conjuctivitis, also they might not be appropriate for this bout. You might try calling your ophthalmologist and telling him you have the same symptoms again and maybe he will call in another bottle of drops but he may also want to see you as there are many things that can mimic "pink eye" or conjuctivitis. You could have a corneal abraision or ulcer. A corneal ulcer can be detrimental to your sight if it goes untreated so to be on the safe side, he may want to see you again. It could also be from allergies or viral causes, in which an antibiotic would not be indicated. Conjuctivitis is very contagious so be careful not to let anyone else use the same towels and cloths that you use near your eyes. Be sure to wash your hands thoroughly after touching near your eyes.

Yes, you could use it since the med does not expire until December 9th.