Hello and thanks for investigating this important health subject!
Gentamicin is known to cause temporary hearing loss in some dogs. Unfortunately, it is one of the best medications known to eliminate bacterial growth in the ears so it has not been replaced as a standard medication to use. An inflammation of the sound-sensing hairs inside the ear canal would be to blame in this type of case.
Any pet experiencing an adverse drug effect should not be exposed to that medication again in future:
http://www.petplace.com/drug-library/gentamicin-gentocin/page1.aspx
It can take anywhere from 2-4 weeks for hearing to return once the medication has been withdrawn. If 3 weeks pass with no improvement, you may need to consider asking your regular vet for a referral to a Veterinary Neurologist to have a complete analysis of your pet's hearing loss.
***But first, I would have your regular vet re-examine these ears. Wax build up can interfere with the normalizing process. Your vet can look all the way down the canal to the ear drums and tell you if they are healthy, or if an ear drum has a tear in it.***
Here is a link that has a lot of information about hearing loss in dogs and some ideas about how to help at home:
http://www.petplace.com/dogs/deafness-in-dogs/page1.aspx
http://www.petplace.com/dogs/how-to-live-with-a-deaf-dog/page1.aspx
If this was caused by the medication, your patience and gentle care are the best friends your dog has at this time!
The Bayer Animal Health Corp. makes an ear treatment that is based on the antibiotic enrofloxacin (called Baytril Otic), this would probably be your next best choice for the future.
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