I am so sorry to hear about your pup's repeated, chronic ear infections. I understand that you are very frustrated because this seems to be a never ending expense. That's often because the underlying problems that lead to ear infections are probably inherent in your pup, and if we do not address those the infections will return or never really clear.
In general oral medications alone will not successfully treat an ear infection. There are some that can help prevent some of the predisposing factors for developing infections, and thus are useful for prevention, but it is extremely rare to be able to give an oral medication and cure an ear infection.
Ear infections are often related to allergies, either food allergies or inhaled allergies.
However are we sure that her original infection completely cleared? Sometimes an infection will look cleared just by looking but infectious agents still linger and if we stop without checking an ear cytology to make sure she is cleared the infection can return quickly after treatment because it was never truly gone. I recommend checking a cytology and then if negative treating for another 5 to 7 days to make sure.
Food allergy is very possible with her as food allergic dogs often have very itchy ears. Dogs can develop allergies to any protein or carbohydrate so even if she is only fed one thing that can be what she is allergic to. Dogs with food allergies tend to lick and scratch their paws, face and ears the most, but any of the "allergy reactive areas" can be affected, and sometimes the only symptom can be repeated ear infections. You could try a true hypoallergenic diet like Hills z/d or Purina Veterinary Diets HA. No treats, flavored medication or bones while on the diet and it must be used for a least 12 to 16 weeks to see the full effects. Most clients do report some improvement in 4 to 6 weeks.
Over the counter foods may be labeled hypoallergenic but they are unlikely to truly be so.
The trouble with "limited ingredient", "hypoallergenic" or "low allergy" or grain free pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your dog isn't allergic to those ingredients but a waste of money thinking that the food was hypoallergenic, and not good for your dog if those happen to be allergens for your dog.
The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food or to hydrolyze the proteins in the food. And the only ingredients in that food, even at a trace level, are what is listed on the bag.
Even though some pet food companies may try to convince you otherwise there are no magic hypoallergenic food ingredients. A food allergy reaction is based upon the dog's body recognizing an allergen, so they must have been exposed to it before. Dogs can be allergic to proteins as well as grains. Prescription foods are hydrolyzed, or broken down so the body cannot recognize the allergen, or use very unusual ingredients that the dog cannot have been exposed to in the past. They cannot develop an allergy to something they have never been exposed to.
If her ears seem to worsen seasonally then her allergies can be inhaled too (like grass pollen, dust mites or molds) and you can use a combination of antihistamines and high doses of omega-3 fatty acids to help prevent symptoms of those types of allergic reactions (they also help with the symptoms of flea allergy). In combination fatty acids and antihistamines work synergistically, much better than either one alone.
You can try:
1)Benadryl (diphenhydramine only don't use the combination products with decongestants or acetaminophen as they can be toxic) at a dose of 1mg to 2mg per pound or one 25mg capsule per 15-25 pounds of body weight orally every 8 hours.
OR 2)Claritin (loratidine) at 5mg per 25 pound dog once or twice daily.
OR 3)Hydroxyzine at 1mg per pound of body weight orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
OR 5) Zyrtec (Cetirizine hydrochloride) at 1/2 mg per pound of body weight orally every 24 hours. That would be one 10mg tablet per 20 pounds of body weight. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because dogs cannot tolerate decongestants.
Some dogs do better on one antihistamine rather than another.
You'll have to see which one works. Give the one that you pick a week trial and if it isn't working try another. Be aware that antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight, meaning if he is between doses use the higher dose. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example an 80 pound dog could take 1600mg of EPA per day.
I also recommend using a gentle ear cleaner twice a week at least. This removes excess wax and keeps the ear pH at levels that discourage infections. It also allows you to know what a normal ear looks like and catch ear infections very early. Sometimes with an early infection simply cleaning the ear daily is enough to stop it in its tracks.
If all of that isn't enough sometimes I will use a topical ear anti-inflammatory such as Synotic 2 to 3 times as week to prevent inflammation and the resulting ear infection.
Please feel free to respond with further questions.