Sometimes they can though they are not really made for ear use, but a product I've found that works really well on persistent infections is called Zymox with cortisone. You can read about it here
http://animalpetdoctor.homestead.com/Zymox.html
The last time I got that www.amazon.com had the best price including shipping but local pet supply stores may also carry it.
I've found it works better for long term infections or allergy based yeast infections than anything else I've used.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics