World-renowned veterinarian, Dr. Stanley Marks, has very generously answered my questions on the dangers of using low-dose tylosin (tylan) and tetracycline to control tear stains. If you're using an antibiotic, or an antibiotic product like Angel Eyes or Angel's Glow, I urge you to read on.
 

My Question:  Dr. Marks, I am researching the long term use of low-dose tylosin by people trying to rid their dog of tear stains. I’m told you use tylosin a great deal for chronic canine diarrhea and know more about it than anyone. To clear up tear stains, people give this antibiotic (and also tetracycline) in unknown strengths for years and sometimes even for the life of the dog. I am concerned that not only aren't underlying causes of epiphora [excess tearing] being addressed, but also that antibiotic resistance and dysbiosis may result. I'd greatly appreciate your thoughts on long-term use of tylosin for cosmetic purposes.

The veterinarian: Dr. Stanley L. Marks, BVSc, PhD, Board certified in Internal Medicine (subspecialties of Internal Medicine and Oncology) and Veterinary Nutrition; Professor, Department of Medicine and Epidemiology UC Davis School of Veterinary Medicine: Director of the Companion Animal Gastrointestinal Laboratory; Speaker of the Year: 2003 North American Veterinary Conference

Note that I added the highlighting in his response for emphasis of important points.

Response from Dr. Marks:  The notion of using tylosin to eradicate tear stains is a bizarre and concerning application of this antibiotic. Your concerns regarding the long-term repercussions are spot-on -- emerging antibiotic resistance (especially when using suboptimal doses of the drug) and potentially dysbiosis.

We use tylosin for antibiotic-responsive diarrhea (ARD) in dogs, for the management of colitis, and for the management of Clostridium perfringens-associated diarrhea. In each of these maladies, we use the recommended dose of drug for the shortest possible time. This can be a challenge with ARD, as many dogs need prolonged administration of the drug for this specific condition.

We have looked at antibiotic resistance against tylosin in dogs with C. perfringens-associated diarrhea, and the good news is that the incidence is extremely low at the moment. This however is likely to change with suboptimal dosing of drug given for months to years.

The use of tetracycline for the clearing of tear stains is even more concerning, as this specific antibiotic has been associated with a very high incidence of antibiotic resistance in people, and we were able to show that 21% of canine C. perfringens isolates were resistant to this antibiotic. Of greater concern, tetracyclines have been associated with the conjugative transfer of resistant plasmids from one bacterium to another unrelated bacterium! *

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* Important: If bacteria A learns how to resist, say, tetracycline, it can pass this information to other bacteria who may also learn to resist it. This can render the tetracycline ineffective in multiple situations. Wikipedia says: "If a bacterium carries several resistance genes, it is called multiresistant or, informally, a superbug."