My opinion on supplements has evolved in the 35 years I've been in practice. I used to be of the opinion that Jayco expressed, basically if you feed a good well balanced diet, supplementation was a waste.
In healthy middle aged animals that's probably still the case, though a good multiple vitamin is pretty cheap insurance for meeting minimum needs.
In the very young and in geriatric patients and those with specific disease conditions supplementation can be very beneficial and tremendously improve quality and probably length of life.
To me the question is not prescription OR OTC supplements, rather what is the best use of what's available to achieve desired results. This might mean only supplements or a combination of many different supplements and prescription meds. Often the progression of osteoarthritis means adding multiple medications to maintain a reasonable quality of life.
Glucosamein/Chondroitin is one of the best supplements for orthopedic conditions. There are so many brands of products that giving recommendations is difficult. We use Nutramax products on Charmin (Cosequin or Dasuquin), and Synovi G3, or Adroitin on Jill. The difference in pet products is that many are flavored for ease of administration and the formulation is a little bit different than human versions. BUT, I agree with Lee that human versions are OK if you can get your dog/cat to take them and you can calculate the correct dose.
I have not seen data on ocular supplements being beneficial for general eye problems we usually encounter in dogs. Lutein is recommended in people to prevent macular degeneration which is not a common problem in dogs. I'd ask the veterinary ophthalmologist you deal with about these supplements.
One other note on nutraceuticals: they are much less regulated by govnt. agencies. Studies have shown that many do not contain the level of medication that the bottles indicate. The higher priced brands don't always indicate better product. The reason we use/recommend the brands we do is that they seem to have more consistent results.
Doug, DVM
Doug & Sandy
Kaylee
Winnie 6 1/2 year old golden
2008 Southwind 2009 Honda CRV